diff --git a/doc/source/audiogenerators.rst b/doc/source/audiogenerators.rst
index 337da5e..25efa81 100644
--- a/doc/source/audiogenerators.rst
+++ b/doc/source/audiogenerators.rst
@@ -7,6 +7,9 @@ mpd
 picks ``howmany`` song randomly from your mpd library. It follows this
 strategy: it picks ``howmany`` artists from your MPD library, then picks a random song for each one
 
+if you specify a ``prefix``, then only files inside the ``prefix`` directory
+will be picked.
+
 randomdir
 ----------