[Sync EN] Document that readonly properties cannot be initialized by reference (#5226)

[github-actions]

Commit: https://github.com/php/doc-en/commit/4e77868e15fea65175b87a38d6d31b61d63f7f8e
PR: https://github.com/php/doc-en/pull/5226

Archivos ES a actualizar

• language/oop5/properties.xml