drm/doc: integrate fb helper reference into docs

Again only minimal changes to make kerneldoc no longer shout. Plus a
little introduction in the form of a inline DOC: section to quickly
explain what this is all about.

v2: Fixup spelling fail.

Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>