loadfile: fix --no-subs-with-matching-audio with --slang

If --slang was set to some language and it matched the subtitle track,
then --no-subs-with-matching-audio would do nothing. Fix the logic by
doing the --no-subs-with-matching-audio step at the end to ensure that
it always "wins" over whatever --slang or --subs-fallback has set.
Clarify the docs a bit to make it clearer that this is the intended
behavior. Fixes fbe8f9919428a7ed24a61899bfd85bbb7680e389.