m3u: Update docs.

docs/ext/m3u.rst

@@ -54,3 +54,14 @@ See :ref:`config` for general help on configuring Mopidy.
 
     Path to directory with M3U files. Unset by default, in which case the
     extension's data dir is used to store playlists.
+
+.. confval:: m3u/default_encoding
+
+    Text encoding used for files with extension ``.m3u``.  Default is
+    ``latin-1``. Note that files with extension ``.m3u8`` are always
+    expected to be UTF-8 encoded.
+
+.. confval:: m3u/default_extension
+
+    The file extension for M3U playlists created using the core playlist
+    API. Default is ``.m3u8``.

mopidy/backend.py

@@ -347,13 +347,14 @@ class PlaylistsProvider(object):
         """
         Create a new empty playlist with the given name.
 
-        Returns a new playlist with the given name and an URI.
+        Returns a new playlist with the given name and an URI, or :class:`None`
+        on failure.
 
         *MUST be implemented by subclass.*
 
         :param name: name of the new playlist
         :type name: string
-        :rtype: :class:`mopidy.models.Playlist`
+        :rtype: :class:`mopidy.models.Playlist` or :class:`None`
         """
         raise NotImplementedError