Normally, OSD can be disabled with --osd-level=0. But this also disables
terminal OSD, and some users want _only_ the terminal OSD. Add
--video-osd=no, which essentially disables the video OSD.
Ideally, it should probably be possible to control terminal and video
OSD levels independently, but that would require separate OSD timers
(and other state) for both components, so don't do it. But because the
current situation isn't too ideal, add a threat to the manpage that
might be changed in the future.