doc(guide/Animations): point to $animate.pin() to enable animation for elements outside an Angular app

Add a section inside of the ngAnimate documentation to point to $animate.pin().
It was not clear in the documentation why animations were being disabled inside of
modals and popup menus. This documentation explains in what situations elements may
be dynamically placed outside of the application DOM and points to the API documentation
on how to enable animations for these types of elements.

Related #14907
Closes #14918

Author: John (J5) Palmieri

docs/content/guide/animations.ngdoc

@@ -347,6 +347,7 @@ By setting `transition: 0s`, ngAnimate will ignore the existing transition style
 animations will still execute, though). This can be used to prevent {@link guide/animations#preventing-collisions-with-existing-animations-and-third-party-libraries
 issues with existing animations interfering with ngAnimate}.
 
+
 ## Preventing flicker before an animation starts
 
 When nesting elements with structural animations such as `ngIf` into elements that have class-based
@@ -419,6 +420,23 @@ than any styles that are independent from ngAnimate.
 
 You can also use one of the two other {@link guide/animations#how-to-selectively-enable-disable-and-skip-animations strategies to disable animations}.
 
+
+### Enable animations for elements outside of the Angular application DOM tree: {@link ng.$animate#pin $animate.pin()}
+
+Before animating, `ngAnimate` checks to see if the element being animated is inside the application DOM tree,
+and if it is not, no animation is run. Usually, this is not a problem as most apps use the `ngApp`
+attribute / bootstrap the app on the `html` or `body` element.
+
+Problems arise when the application is bootstrapped on a different element, and animations are
+attempted on elements that are outside the application tree, e.g. when libraries append popup and modal
+elements as the last child in the body tag.
+
+You can use {@link ng.$animate#pin `$animate.pin(elementToAnimate, parentHost)`} to specify that an
+element belongs to your application. Simply call it before the element is added to the DOM / before
+the animation starts, with the element you want to animate, and the element which should be its
+assumed parent.
+
+
 ## More about animations
 
 For a full breakdown of each method available on `$animate`, see the {@link ng.$animate API documentation}.