Update man page for fonts.conf and subfont.ttf. These are two
undocumented features in mpv. They were only hardcoded into sub/ass_mp.c
and could not be found anywhere else in the entire codebase. Git log
reveals that fonts.conf was added in 2013 while subfont.ttf was brought
in by a ancient patch of mplayer in 2002...
These are two quite useful undocumented features when you do not want to
mess up with global fonts.conf to include more fonts.
Also document ~/.config/mpv/fonts/ directory and suggest using fonts.conf
to include additional fonts.
mpv reads all files in ~/.config/mpv/fonts/ directory into memory. If
there are a lot of fonts in that directory, mpv would use a lot of
memory. Using ~/.config/mpv/fonts.conf to include additional fonts is
more memory-efficent.