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

drivers/gpu/drm/drm_connector.c

@@ -1397,6 +1397,8 @@ EXPORT_SYMBOL(drm_connector_attach_dp_subconnector_property);
  *	Game:
  *		Content type is game
  *
+ *	The meaning of each content type is defined in CTA-861-G table 15.
+ *
  *	Drivers can set up this property by calling
  *	drm_connector_attach_content_type_property(). Decoding to
  *	infoframe values is done through drm_hdmi_avi_infoframe_content_type().