drm/connector: document enum drm_connector_tv_mode DRM_MODE_TV_MODE_MAX

Add documentation for the DRM_MODE_TV_MODE_MAX enumerator to fix the
kernel-doc warning:

include/drm/drm_connector.h:204: warning: Enum value 'DRM_MODE_TV_MODE_MAX' not described in enum 'drm_connector_tv_mode'
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Link: https://patchwork.freedesktop.org/patch/msgid/20230504123444.1843795-1-jani.nikula@intel.com

include/drm/drm_connector.h

@@ -199,6 +199,11 @@ enum drm_connector_tv_mode {
 	 */
 	DRM_MODE_TV_MODE_SECAM,
 
+	/**
+	 * @DRM_MODE_TV_MODE_MAX: Number of analog TV output modes.
+	 *
+	 * Internal implementation detail; this is not uABI.
+	 */
 	DRM_MODE_TV_MODE_MAX,
 };