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 fbe8f99194.

DOCS/man/options.rst

@@ -137,7 +137,9 @@ Track Selection
 
 ``--subs-with-matching-audio=<yes|no>``
     When autoselecting a subtitle track, select a full/non-forced one even if the selected
-    audio stream matches your preferred subtitle language (default: yes).
+    audio stream matches your preferred subtitle language (default: yes). If this option is
+    set to ``no``, a non-forced subtitle track that matches the audio language will never be
+    autoselected by mpv regardless of the value of ``--slang`` or ``--subs-fallback``.
 
 ``--subs-match-os-language=<yes|no>``
     When autoselecting a subtitle track, select the track that matches the language of your OS