man: document fonts.conf, subfont.ttf, and fonts subdir

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.

DOCS/man/mpv.rst

@@ -996,6 +996,23 @@ For Windows-specifics, see `FILES ON WINDOWS`_ section.
 ``~/.config/mpv/input.conf``
     key bindings (see `INPUT.CONF`_ section)
 
+``~/.config/mpv/fonts.conf``
+    Fontconfig fonts.conf that is customized for mpv. You should include system
+    fonts.conf in this file or mpv would not know about fonts that you already
+    have in the system.
+
+    Only available when libass is built with fontconfig.
+
+``~/.config/mpv/subfont.ttf``
+    fallback subtitle font
+
+``~/.config/mpv/fonts/``
+    Font files in this directory are used by mpv/libass for subtitles. Useful
+    if you do not want to install fonts to your system. Note that files in this
+    directory are loaded into memory before being used by mpv. If you have a
+    lot of fonts, consider using fonts.conf (see above) to include additional
+    fonts, which is more memory-efficient.
+
 ``~/.config/mpv/scripts/``
     All files in this directory are loaded as if they were passed to the
     ``--script`` option. They are loaded in alphabetical order, and sub-directories