options: accept --audio-channels=auto

This sounds much more intuitive, while "empty" was a bit of a WTF.

DOCS/man/options.rst

@@ -907,8 +907,9 @@ Audio
     lists speaker names, which can be used to express arbitrary channel
     layouts (e.g. ``fl-fr-lfe`` is 2.1).
 
-    You can use ``--audio-channels=empty`` to disable this. In this case, the AO
-    use the channel layout as the audio filter chain indicates.
+    You can use ``--audio-channels=auto`` to disable this. In this case, the AO
+    use the channel layout as the audio filter chain indicates. (``empty`` is
+    an accepted obsolete for ``auto``.)
 
     This will also request the channel layout from the decoder. If the decoder
     does not support the layout, it will fall back to its native channel layout.

audio/chmap.c

@@ -86,6 +86,7 @@ static const char *const std_layout_names[][2] = {
     {"7.1(wide-side)",  "fl-fr-fc-lfe-flc-frc-sl-sr"},
     {"octagonal",       "fl-fr-fc-bl-br-bc-sl-sr"},
     {"downmix",         "dl-dr"},
+    {"auto",            ""}, // not in lavc
     {0}
 };