Bug 858192 - Correct error in nsIContentPrefService2 documentation about callbacks and observers. r=mak

dom/interfaces/base/nsIContentPrefService2.idl

@@ -300,8 +300,9 @@ interface nsIContentPrefService2 : nsISupports
    * Registers an observer that will be notified whenever a preference with the
    * given name is set or removed.
    *
-   * When a set or remove method is called, observers are notified after the set
-   * or removal completes but before method's callback is called.
+   * When a set or remove method is called, observers are called after the set
+   * or removal completes and after the method's callback is called, and they
+   * are called in the same turn of the event loop as the callback.
    *
    * The service holds a strong reference to the observer, so the observer must
    * be removed later to avoid leaking it.