docs: Add note on what the backend API docs covers. Add graph of backend controller relations.

docs/api/backends.rst

@@ -1,6 +1,30 @@
-*************************************
-:mod:`mopidy.backends` -- Backend API
-*************************************
+**********************
+:mod:`mopidy.backends`
+**********************
+
+The backend and its controllers
+===============================
+
+.. graph:: backend_relations
+
+    backend -- current_playlist
+    backend -- library
+    backend -- playback
+    backend -- stored_playlists
+
+
+Backend API
+===========
+
+.. note::
+
+    Currently this only documents the API that is available for use by
+    frontends like :class:`mopidy.mpd.handler`, and not what is required to
+    implement your own backend. :class:`mopidy.backends.BaseBackend` and its
+    controllers implements many of these methods in a matter that should be
+    independent of most concrete backend implementations, so you should
+    generally just implement or override a few of these methods yourself to
+    create a new backend with a complete feature set.
 
 .. automodule:: mopidy.backends
     :synopsis: Backend interface.