drm: kerneldoc polish for drm_crtc_helper.c

Most of this is newly added kerneldoc for the hotplug and output
polling code. But I've also thrown in a bit lesser polish, most of it
is tuning down the shouting RETURN: headers.

Overview documentation for the output probing and mode setting support
code will be added in later patches.

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