Updates `GoogleMapController.clearTileCache` documentation to explain that some platform implementations may remove current tiles before replacements are available, which can cause the tile overlay to flicker. The wording remains implementation-neutral so it also applies to third-party platform implementations.

Fixes flutter/flutter#189678.

## Pre-Review Checklist

Test exemption: This PR changes only API documentation and the required release metadata; it does not change runtime behavior.

Validation:
- `dart format --output=none --set-exit-if-changed packages/google_maps_flutter/google_maps_flutter/lib/src/controller.dart`
- `dart run script/tool/bin/flutter_plugin_tools.dart analyze --packages=google_maps_flutter/google_maps_flutter --exact-match-only`
- `dart run script/tool/bin/flutter_plugin_tools.dart dart-test --packages=google_maps_flutter/google_maps_flutter --exact-match-only`
- `dart run script/tool/bin/flutter_plugin_tools.dart validate --packages=google_maps_flutter/google_maps_flutter --exact-match-only --base-branch=upstream/main`
- `dart run script/tool/bin/flutter_plugin_tools.dart publish-check --packages=google_maps_flutter/google_maps_flutter --exact-match-only`

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