libsaria: Add comment to playlist save()

The lock should be held while saving.  Save should normally be called by
a function that already holds the lock, though.

libsaria/sources/playlist/playlist.py

@@ -19,6 +19,7 @@ def load():
 	song_list = plist_list
 	unlock_plist()
 
+# NOTE: You should already be holding the lock when you call this
 def save():
 	storage.save_obj("playlist", song_list)