docs($compile): Explain that post-link functions run in reverse order.

Update the $compile docs to mention the change introduced in angular#4266.

Closes angular#4843

Author: Martin Field

src/ng/compile.js

@@ -115,8 +115,9 @@
  * When there are multiple directives defined on a single DOM element, sometimes it
  * is necessary to specify the order in which the directives are applied. The `priority` is used
  * to sort the directives before their `compile` functions get called. Priority is defined as a
- * number. Directives with greater numerical `priority` are compiled first. The order of directives with
- * the same priority is undefined. The default priority is `0`.
+ * number. Directives with greater numerical `priority` are compiled first. Pre-link functions
+ * are also run in priority order, but post-link functions are run in reverse order. The order
+ * of directives with the same priority is undefined. The default priority is `0`.
  *
  * #### `terminal`
  * If set to true then the current `priority` will be the last set of directives