drm/doc: Clarify connector overview

There was a bit of mix-up between initialization and registering.

v2: Review from Gustavo.

Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170125062657.19270-7-daniel.vetter@ffwll.ch
Link: http://patchwork.freedesktop.org/patch/msgid/20170125062657.19270-8-daniel.vetter@ffwll.ch