drm/doc: Improve docs around connector (un)registration

Current code is quite a mess unfortunately, so also add a todo.rst
entry to maybe fix it up eventually.

Cc: Michel Dänzer <michel@daenzer.net>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190917120936.7501-2-daniel.vetter@ffwll.ch