docs(ngAnimate): add animating any scope's variable change section

Add a section which covers use case when user's need to animate upon every variable's value change.

Refers angular#16561

Author: FRSgit

src/ngAnimate/module.js

@@ -267,9 +267,14 @@
  * .message.ng-enter-prepare {
  *   opacity: 0;
  * }
- *
  * ```
  *
+ * ### Animating any scope's variable change
+ *
+ * Unless possible with more proper ["animation aware" directive](#directive-support), that specific use case can always be covered with {@link ngAnimate.directive:ngAnimateSwap} as can be seen in {@link ngAnimate.directive:ngAnimateSwap#examples this example}.
+ *
+ * Note that {@link ngAnimate.directive:ngAnimateSwap} is a *structural directive*, which means it creates a new instance of the element (including any other/child directives it may have) and links it to a new scope every time *swap* happens. In some cases this might not be desirable (e.g. for performance reasons, or when wish to retain internal state on the original element's instance).
+ *
  * ## JavaScript-based Animations
  *
  * ngAnimate also allows for animations to be consumed by JavaScript code. The approach is similar to CSS-based animations (where there is a shared