Doc improvements for animations (#14892)

* Doc improvements for animations

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

@@ -638,6 +638,10 @@ class AnimationMean extends CompoundAnimation<double> {
 /// The [value] of this animation is the maximum of the values of
 /// [first] and [next].
 class AnimationMax<T extends num> extends CompoundAnimation<T> {
+  /// Creates an [AnimationMax].
+  ///
+  /// Both arguments must be non-null. Either can be an [AnimationMax] itself
+  /// to combine multiple animations.
   AnimationMax(Animation<T> first, Animation<T> next): super(first: first, next: next);
 
   @override
@@ -649,6 +653,10 @@ class AnimationMax<T extends num> extends CompoundAnimation<T> {
 /// The [value] of this animation is the maximum of the values of
 /// [first] and [next].
 class AnimationMin<T extends num> extends CompoundAnimation<T> {
+  /// Creates an [AnimationMin].
+  ///
+  /// Both arguments must be non-null. Either can be an [AnimationMin] itself
+  /// to combine multiple animations.
   AnimationMin(Animation<T> first, Animation<T> next): super(first: first, next: next);
 
   @override