doc($compile): clarify compile function return value

If a compile function (within a directive) returns a function, it is a
post-link function.

Closes: #2713

Author: Jens Rantil

docs/content/guide/directive.ngdoc

@@ -480,8 +480,8 @@ done in a linking function rather than in a compile function.
 
 A compile function can have a return value which can be either a function or an object.
 
-* returning a function - is equivalent to registering the linking function via the `link` property
-  of the config object when the compile function is empty.
+* returning a (post-link) function - is equivalent to registering the linking function via the
+  `link` property of the config object when the compile function is empty.
 
 * returning an object with function(s) registered via `pre` and `post` properties - allows you to
   control when a linking function should be called during the linking phase. See info about