Fixed #18617 -- Highlighted that the app_directories template loader depends on the order of INSTALLED_APPS.

Thanks evildmp for the patch.

docs/ref/templates/api.txt

@@ -649,14 +649,24 @@ class. Here are the template loaders that come with Django:
 
         INSTALLED_APPS = ('myproject.polls', 'myproject.music')
 
-    ...then ``get_template('foo.html')`` will look for templates in these
+    ...then ``get_template('foo.html')`` will look for ``foo.html`` in these
     directories, in this order:
 
-    * ``/path/to/myproject/polls/templates/foo.html``
-    * ``/path/to/myproject/music/templates/foo.html``
+    * ``/path/to/myproject/polls/templates/``
+    * ``/path/to/myproject/music/templates/``
 
-    Note that the loader performs an optimization when it is first imported: It
-    caches a list of which :setting:`INSTALLED_APPS` packages have a
+    ... and will use the one it finds first.
+
+    The order of :setting:`INSTALLED_APPS` is significant! For example, if you
+    want to customize the Django admin, you might choose to override the
+    standard ``admin/base_site.html`` template, from ``django.contrib.admin``,
+    with your own ``admin/base_site.html`` in ``myproject.polls``. You must
+    then make sure that your ``myproject.polls`` comes *before*
+    ``django.contrib.admin`` in :setting:`INSTALLED_APPS`, otherwise
+    ``django.contrib.admin``'s will be loaded first and yours will be ignored.
+
+    Note that the loader performs an optimization when it is first imported:
+    it caches a list of which :setting:`INSTALLED_APPS` packages have a
     ``templates`` subdirectory.
 
     This loader is enabled by default.