[animations] Clarify that onClosed is called before transition

[adil192]

Updated the documentation for OpenContainer.onClosed to clarify that it is called at the start of the closing transition, not at the end.

Resolves flutter/flutter#178314

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under.
• I updated/added any relevant documentation (doc comments with ///).
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Footnotes

1. This PR only affects comments/documentation. ↩

[gemini-code-assist]

Code Review

This pull request updates the documentation for OpenContainer.onClosed to clarify that the callback is executed at the start of the closing transition, not at the end. The change correctly reflects the component's behavior. The version in pubspec.yaml and the CHANGELOG.md have been updated accordingly. I have one suggestion to make the documentation even more explicit.