[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.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• 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 ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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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.

[hannah-hyj]

LGTM

[justinmc]

LGTM 👍

[loic-sharma]

Suggested change

	/// Called when the container is popped. This is called at the start of the
	/// closing transition.
	/// Called when the container was popped and will return to the closed state.
	///
	/// This is called at the start of the closing transition.