[postprocessor:ugoira] auto-select demuxer by default

docs/configuration.rst

@@ -3239,7 +3239,7 @@ ugoira.ffmpeg-demuxer
 Type
     ``string``
 Default
-    ``concat``
+    ``auto``
 Description
     FFmpeg demuxer to read and process input files with. Possible values are
 
@@ -3247,6 +3247,9 @@ Description
     * "`image2 <https://ffmpeg.org/ffmpeg-formats.html#image2-1>`_" (accurate timecodes, not usable on Windows)
     * "mkvmerge" (accurate timecodes, only WebM or MKV, requires `mkvmerge <ugoira.mkvmerge-location_>`__)
 
+    `"auto"` will select `mkvmerge` if possible and fall back to `image2` or
+    `concat` depending on the local operating system.
+
 
 ugoira.ffmpeg-location
 ----------------------