drm/connector: refer to CTA-861-G in the "content type" prop docs

drm/connector: refer to CTA-861-G in the "content type" prop docs

The KMS documentation doesn't say much about the meaning of each
content type. Add a reference to the specification defining them.

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211004091236.82010-1-contact@emersion.fr