docs: describe rsync hash regex option

docs/Administrator-Guide/Tuning-Volume-Options.md

@@ -48,6 +48,7 @@ description and default value:
 | cluster.lookup-optimize                       | This option enables the optimization of -ve lookups, by not doing a lookup on non-hashed subvolumes for files, in case the hashed subvolume does not return any result. This option disregards the lookup-unhashed setting, when enabled.                                                                                                                                                                                                                                                                                                                                                                                                                                     | on                                                                                                                                                                                                                                                                                                                                                    | on/off                                                                                |
 | cluster.randomize-hash-range-by-gfid          | Allows to use gfid of directory to determine the subvolume from which hash ranges are allocated starting with 0. Note that we still use a directory/file’s name to determine the subvolume to which it hashes                                                                                                                                                                                                                                                                                                                                                                                                                                                                 | off                                                                                                                                                                                                                                                                                                                                                   | on/off                                                                                |
 | cluster.rebal-throttle                        | Sets the maximum number of parallel file migrations allowed on a node during the rebalance operation. The default value is normal and allows 2 files to be migrated at a time. Lazy will allow only one file to be migrated at a time and aggressive will allow maxof[(((processing units) - 4) / 2), 4]                                                                                                                                                                                                                                                                                                                                                                      | normal                                                                                                                                                                                                                                                                                                                                                | lazy/normal/aggressive                                                                |
+| cluster.rsync-hash-regex                      | POSIX extended regular expression used to strip rsync temporary-file prefixes and suffixes, preventing relocation when a file is renamed. The built-in default is `^\\.(.+)\\.[^.]+$`. Use `none` to disable the optimization. Invalid expressions are logged and ignored. To avoid backslash escaping in a volfile, use a character class such as `[.]`; for example, `^[.].+[.][^.]+$` matches names such as `.file.txt.123456`.                                                                                                                                                       | built-in default                                                                                                                                                                                                                                                                                                                                       | POSIX extended regular expression or `none`                                         |
 | cluster.background-self-heal-count            | Specifies the number of per client self-heal jobs that can perform parallel heals in the background.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | 8                                                                                                                                                                                                                                                                                                                                                     | 0-256                                                                                 |
 | cluster.heal-timeout                          | Time interval for checking the need to self-heal in self-heal-daemon                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | 600                                                                                                                                                                                                                                                                                                                                                   | 5-(signed-int)                                                                        |
 | cluster.eager-lock                            | If eager-lock is off, locks release immediately after file operations complete, improving performance for some operations, but reducing access efficiency                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     | on                                                                                                                                                                                                                                                                                                                                                    | on/off                                                                                |