docs($animateCss): add missing documentation for the structural option

Add missing documentation for structural option in `$animateCss` service

Closes #13049

src/ngAnimate/animateCss.js

@@ -186,6 +186,8 @@ var ANIMATE_TIMER_KEY = '$$animateCss';
  *
  * * `event` - The DOM event (e.g. enter, leave, move). When used, a generated CSS class of `ng-EVENT` and `ng-EVENT-active` will be applied
  * to the element during the animation. Multiple events can be provided when spaces are used as a separator. (Note that this will not perform any DOM operation.)
+ * * `structural` - Indicates that the `ng-` prefix will be added to the event class. Setting to `false` or omitting will turn `ng-EVENT` and
+ * `ng-EVENT-active` in `EVENT` and `EVENT-active`. Unused if `event` is omitted.
  * * `easing` - The CSS easing value that will be applied to the transition or keyframe animation (or both).
  * * `transitionStyle` - The raw CSS transition style that will be used (e.g. `1s linear all`).
  * * `keyframeStyle` - The raw CSS keyframe animation style that will be used (e.g. `1s my_animation linear`).