drm/doc: Clarify the dumb object interfaces

drm/doc: Clarify the dumb object interfaces

- This is _not_ a generic interface to create gem objects, but just an
  interface to make early boot services (like boot splash) with a
  generic KMS userspace driver possible. Hence it's better to move
  the documentation for this from the GEM section to the KMS section,
  next to the creation of framebuffer objects.

- Make it really clear that the returned handle isn't necessarily a
  GEM object (it can also be e.g. a TTM handle when running on top of
  vmwgfx).

- Add a paragraph to make it clear that this is just for unaccelarated
  userspace - gpu drivers need to have their own buffer object
  creation ioctl which is hardware specific.

v2: Clarify that the documentation doesn't just apply to GEM-based
drivers only but is now generally valid, as suggested by David.

v3: Polish the intro sentence a bit and one s/objects/handles/ for
clarification, both suggested by Laurent.

v4: More text polish from Laurent's review.

v5: More typo fixes from Dieter.

Cc: Dieter Nützel <Dieter@nuetzel-hh.de>
Cc: David Herrmann <dh.herrmann@gmail.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>