webrtc: Add hotdoc style since tags

We're stuck having to add a separate comment for now.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1425>

gst-libs/gst/webrtc/rtpsender.h

@@ -48,6 +48,14 @@ GType gst_webrtc_rtp_sender_get_type(void);
  *
  * Since: 1.16
  */
+/**
+ * GstWebRTCRTPSender.priority:
+ *
+ * The priority of the stream.
+ *
+ * Since: 1.20
+ */
+
 struct _GstWebRTCRTPSender
 {
   GstObject                          parent;

gst-libs/gst/webrtc/rtptransceiver.h

@@ -61,6 +61,13 @@ GType gst_webrtc_rtp_transceiver_get_type(void);
  *
  * Since: 1.16
  */
+/**
+ * GstWebRTCRTPTransceiver.kind:
+ *
+ * Type of media
+ *
+ * Since: 1.20
+ */
 struct _GstWebRTCRTPTransceiver
 {
   GstObject                         parent;