Rename the text subtitle options from --sub-text- to --sub-
and --ass- options to --sub-ass-.
The intention is to common sub options to prefixed --sub-
and special ASS option be seen as a special version of sub options.
The OSD options that work like the --sub- options are still named
--osd-.
Man page updated including a short note about renamed --sub-text-*
and --ass-* options to --sub-* and --sub-ass-*.