Clarify overridden_fields lint documentation

[gingerninja85]

Clarifies the published documentation for the overridden_fields lint.

The issue discussion notes a common misconception: users may think overriding a field just changes the superclass field's initializer. This adds a short explanation that the subclass field is a separate storage location and does not replace or reinitialize the superclass field.

Fixes #58311.

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• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Validation:

• Parsed pkg/linter/messages.yaml with PyYAML successfully.
• Ran git diff --check successfully.

Note: I could not run the Dart linter/doc generation tests in this shallow GitHub clone because this environment does not have a Dart SDK binary or the full gclient fetch checkout required by CONTRIBUTING.md.

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