manpage: document why Vorbis backward playback does not work

The only reasonable solution to this is probably to make discarding of
preroll frames based on timestmaps, instead of frame/packet count. But
then you get issues with video and its dumb timestamp reordering. So for
now, fuck it.

DOCS/man/options.rst

@@ -489,6 +489,13 @@ Playback Control
       A workaround is to remux to a format like mkv, which enforces packet
       boundaries. Making mpv cache the entire file in memory also works.
 
+    - Backward playback with Vorbis does not work. libavcodec's decoder
+      discards the first Vorbis packet (after each decoder reset), and the
+      mechanism behind ``--audio-reversal-buffer`` assumes that it strictly
+      outputs a frame for each packet fed to it (the mechanism discards output
+      based on frame count, not timestamps). Since this tries to decode each
+      frame on its own, the player never gets a usable audio frame.
+
     Tuning:
 
     - Remove all ``--vf``/``--af`` filters you have set. Disable deinterlacing.