Improve documentation around animations. (#42267)

packages/flutter/lib/src/animation/animation_controller.dart

@@ -673,6 +673,10 @@ class AnimationController extends Animation<double>
 
   /// Drives the animation according to the given simulation.
   ///
+  /// The values from the simulation are clamped to the [lowerBound] and
+  /// [upperBound]. To avoid this, consider creating the [AnimationController]
+  /// using the [new AnimationController.unbounded] constructor.
+  ///
   /// Returns a [TickerFuture] that completes when the animation is complete.
   ///
   /// The most recently returned [TickerFuture], if any, is marked as having been

packages/flutter/lib/src/physics/spring_simulation.dart

@@ -122,8 +122,8 @@ class SpringSimulation extends Simulation {
   String toString() => '$runtimeType(end: $_endPosition, $type)';
 }
 
-/// A SpringSimulation where the value of [x] is guaranteed to have exactly the
-/// end value when the simulation isDone().
+/// A [SpringSimulation] where the value of [x] is guaranteed to have exactly the
+/// end value when the simulation [isDone].
 class ScrollSpringSimulation extends SpringSimulation {
   /// Creates a spring simulation from the provided spring description, start
   /// distance, end distance, and initial velocity.