Fixed #18617 -- Highlighted that the app_directories template loader depends...

        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.