Clarify getAvailableBiometrics documentation

[tranthequang]

Summary

• Clarifies that getAvailableBiometrics returns biometrics available for the app to use.
• Notes that this may not include all enrolled biometrics, such as when iOS permissions are denied.

Fixes flutter/flutter#187512

Tests

• dart format lib/src/local_auth.dart
• flutter analyze
• flutter test

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[gemini-code-assist]

Code Review

This pull request updates the documentation for the getAvailableBiometrics method in local_auth.dart to clarify that it returns biometrics available for the app to use, which may not include all enrolled biometrics if permissions are not granted. There are no review comments, and no additional feedback is provided.

[stuartmorgan-g]

Thanks for the contribution!

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