drm: Update GEM refcounting docs

I just realized that I've forgotten to update all the gem refcounting
docs. For pennance also add pretty docs for the overall drm_gem_object
structure, with a few links thrown in fore good.

As usually we need to make sure the kerneldoc reference is at most a
sect2 for otherwise it won't be listed.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1445533889-7661-1-git-send-email-daniel.vetter@ffwll.ch


Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>