Clarify which mixer is which in al_attach_mixer_to_mixer's documentation
Document that al_set_new_bitmap_format takes ALLEGRO_PIXEL_FORMAT as an argument
Disallow attaching a mixer to itself.

This is somewhat of a corner case that was previous undocumented. I can't think
of a realistic usage for this that wouldn't imply an infinite recursion
somewhere.
Make al_load_ttf_font_stretch return NULL if invalid width/height are passed.

Previously this was undefined behavior, but it seems pointless to keep it that
way. If this feature is supported in the future, it makes a lot more sense to
have a clear signal when this feature is not supported than silently doing
something random.