doc/muxers: Document title tag for attachments

The Matroska muxer has always mapped the title tag to the FileDescription
element for attachments streams since support for writing attachments
was added in commit c7a63a52. This
commit merely documents this fact.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

doc/muxers.texi

@@ -1314,7 +1314,8 @@ The recognized metadata settings in this muxer are:
 
 @table @option
 @item title
-Set title name provided to a single track.
+Set title name provided to a single track. This gets mapped to
+the FileDescription element for a stream written as attachment.
 
 @item language
 Specify the language of the track in the Matroska languages form.