drm: Expand the drm_helper_probe_single_connector_modes() docs

Describe the procedure that drm_helper_probe_single_connector_modes()
uses to do it's work in the kernel-doc comment.

Caveat: Looks like crap and trying to reverse engineer the documentation
tools is not something I want to do.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449782037-19722-3-git-send-email-ville.syrjala@linux.intel.com