Add documents for the required permission

In order to registerContentObserver as other user, callers need to hold
the INTERACT_ACROSS_USERS_FULL permission.

Bug: 206743591
Test: CtsContentTestCases
Change-Id: I2f373a3f064718cc87bdda35a5854b6a6fd2e7aa

core/java/android/content/ContentResolver.java

@@ -2670,6 +2670,9 @@ public abstract class ContentResolver implements ContentInterface {
      * {@link ContentObserver#onChange(boolean, Collection, int, UserHandle)} should be
      * overwritten to get the corresponding {@link UserHandle} for that notification.
      *
+     * <p> If you don't hold the {@link android.Manifest.permission#INTERACT_ACROSS_USERS_FULL}
+     * permission, you can register the {@link ContentObserver} only for current user.
+     *
      * @param uri                  The URI to watch for changes. This can be a specific row URI,
      *                             or a base URI for a whole class of content.
      * @param notifyForDescendants When false, the observer will be notified