[shared_preferences] Document return values of set and remove methods

[binSaed]

The setBool, setInt, setDouble, setString, setStringList, and remove methods on SharedPreferences all return Future<bool>, but the documentation never explains what the return value means. As noted in a prior PR review, the return value is not self-documenting and required reading every platform implementation to understand.

This PR adds dartdoc to each of these methods using the wording suggested in that review:

Returns false if the platform implementation can definitively determine that storing the preference failed. A return value of true indicates either that the value was written successfully, or that the underlying platform API does not report success or failure.

Fixes flutter/flutter#146070

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• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
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Version/CHANGELOG note: documentation-only change; added a CHANGELOG entry under the existing ## NEXT section with no version bump.
Test exemption: documentation-only change (dartdoc comments only, no code changes).

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩²

[gemini-code-assist]

Code Review

This pull request updates the documentation comments for the setBool, setInt, setDouble, setString, setStringList, and remove methods in SharedPreferences to clarify their return value behavior. Specifically, it explains that these methods return false if the platform implementation definitively determines that the operation failed, and true otherwise. There are no review comments, so there is no additional feedback to provide.

[stuartmorgan-g]

Thank you for your contribution! Because of the volume of PRs we receive, we require that new contributors use our checklist to guide them through critical steps in creating a Flutter PR. This PR's description is missing that checklist, so it is being marked as a Draft.

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[stuartmorgan-g]

(In the future, please don't ping people in PR descriptions. This causes pings both when the PR is opened, and also when the PR description becomes the commit message, and then many more times when people update their forks of the repository to include that commit. I've removed username references from the description.)