drm/doc: add new dp helpers into drm DocBook

I didn't bother with documenting the really trivial new "extract
something from dpcd" helpers, but the i2c over aux ch is now
documented a bit.

v2: Clarify the comment for i2c_dp_aux_add_bus a bit.

v3: Fix more spelling fail spotted by Laurent Pinchart.

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>