Document super call order for State.didChangeDependencies - #185945

The doc comment on State.didChangeDependencies is silent about where overrides should place the super.didChangeDependencies() call, even though @mustCallSuper is set on it. The sibling lifecycle hook State.initState already documents this convention, so the two methods read inconsistently and contributors hitting didChangeDependencies for the first time have to guess.

This PR adds a short note to the dartdoc that mirrors the wording on initState, telling implementations to start with a call to the inherited method (super.didChangeDependencies()). It is a documentation-only change to packages/flutter/lib/src/widgets/framework.dart. There is no behavior change, no API change, and no new public surface.

Fixes #28925.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
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• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt. This PR is test-exempt because it only edits documentation text in a dartdoc comment, with no behavioral or API changes.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.