docs(animation): add note about aliases (#2629)

Author: Foxandxss

public/docs/ts/latest/guide/animations.jade

@@ -179,6 +179,14 @@ figure
   are different on enter and leave: the element enters from the left
   and leaves to the right.
 
+.l-sub-section
+  :marked
+    These two common animations have their own aliases:
+  code-example(language="typescript").
+    transition(':enter', [ ... ]); // void => *
+    transition(':leave', [ ... ]); // * => void
+
+:marked
   ## Example: Entering and leaving from different states
 figure
   img(src="/resources/images/devguide/animations/animation_enter_leave_states.gif" alt="Enter and leave animations combined with state animations" align="right" style="width:200px" )