drm/docs: more leftovers from the big vtable documentation pile
Another pile of vfuncs from the old gpu.tmpl xml documentation that
I've forgotten to delete. I spotted a few more things to
clarify/extend in the new kerneldoc while going through this once
more.
v2: Spelling fixes (Thierry).
v3: More spelling fixes and use Thierry's proposal to clarify why
drivers need to validate modes both in ->mode_fixup and ->mode_valid.