manpage: sort --osd-* options

They were more or less grouped by usefulness, but since everything
else in the manpage is sorted alphabetically, it's better to be
consistent and sort these options as well.
sub: add --sub-text-* options to unstyled text subtitles font

Before this commit, the --osd-* options (like --osd-font-size etc.)
configured both the OSD and subtitle font. Make them separate, and add
--sub-text-* options (like --sub-text-size etc.). Now --osd-* affects
the OSD font only, and --sub-text-* unstyled text subtitles only.
manpage: document the dangers of some subtitle options

Setting some subtitle options may lead to incorrect rendering of complex
ASS subtitle scripts, such as displaced signs or visual artifacts. The
user should be made aware that this can happen.

In theory, libass could make using some of these options relatively
safe, but it doesn't.

Note that there are potentially much more options that could in theory
break subtitle rendering, but add a warning only to the most fragile
ones.