docs(guide/directive): clarify directive priority

Fixes angular#2644.

Author: Jens Rantil

docs/content/guide/directive.ngdoc

@@ -332,8 +332,9 @@ compiler}. The attributes are:
 
   * `priority` - 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. Higher `priority` goes
-    first. The order of directives within the same priority is undefined.
+    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`.
 
   * `terminal` - If set to true then the current `priority` will be the last set of directives
     which will execute (any directives at the current priority will still execute