Clarify performacne of SingleTickerProviderStateMixin vs TickerProviderStateMixin (#164870)

Adds the following line to
[TickerProviderStateMixin](https://api.flutter.dev/flutter/widgets/TickerProviderStateMixin-mixin.html):

>Using one [TickerProviderStateMixin] twice is more efficient than two
[SingleTickerProviderStateMixin]s.

This is based on a discussion on Flutter Discord with @chunhtai. From
what I understood, creating multiple `SingleTickerProviderStateMixin`s
is more expensive because you need two different state which is
relatively costly compared to having just one. Not sure if this line
should should be qualified like "Using one [TickerProviderStateMixin]
twice is *generally* more efficient than two
[SingleTickerProviderStateMixin]s" or similar.

This change should be reviewed for correctness as I don't actually
understand enough about the difference between
`TickerProviderStateMixin` and `SingleTickerProviderStateMixin`.

Closes #164869

<details>

<summary>
Example case where this was relevant (click)
</summary>

I was avoiding using `TickerProviderStateMixin` like so:

```dart
    _animationController = AnimationController.unbounded(vsync: this)
      ..addListener(() {
        _updateScale(_animationController.value);
      });
   
    //could have just set `vsync` to `this` if using `TickerProviderStateMixin`
    WidgetsBinding.instance.addPostFrameCallback((_) {
      _verticalAnimationController =
          AnimationController.unbounded(vsync: _verticalController.position.context.vsync)
            ..addListener(() {
              _verticalController.jumpTo(_verticalAnimationController.value);
            });

      _horizontalAnimationController =
          AnimationController.unbounded(vsync: _horizontalController.position.context.vsync)
            ..addListener(() {
              _horizontalController.jumpTo(_horizontalAnimationController.value);
            });
    });
```

Instead of just:

```dart
    _animationController = AnimationController.unbounded(vsync: this)
      ..addListener(() {
        _updateScale(_animationController.value);
      });

    _verticalAnimationController = AnimationController.unbounded(vsync: this)
      ..addListener(() {
        _verticalController.jumpTo(_verticalAnimationController.value);
      });

    _horizontalAnimationController = AnimationController.unbounded(vsync: this)
      ..addListener(() {
        _horizontalController.jumpTo(_horizontalAnimationController.value);
      });
```

</details>

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we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
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packages/flutter/lib/src/widgets/ticker_provider.dart

@@ -301,6 +301,11 @@ mixin SingleTickerProviderStateMixin<T extends StatefulWidget> on State<T>
 /// If you only have a single [Ticker] (for example only a single
 /// [AnimationController]) for the lifetime of your [State], then using a
 /// [SingleTickerProviderStateMixin] is more efficient. This is the common case.
+///
+/// When creating multiple [AnimationController]s, using a single state with
+/// [TickerProviderStateMixin] as vsync for all [AnimationController]s is more
+/// efficient than creating multiple states with
+/// [SingleTickerProviderStateMixin].
 @optionalTypeArgs
 mixin TickerProviderStateMixin<T extends StatefulWidget> on State<T> implements TickerProvider {
   Set<Ticker>? _tickers;