gstwaylandsink: add some locking documentation

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9283>

subprojects/gst-plugins-bad/ext/wayland/gstwaylandsink.c

@@ -219,6 +219,7 @@ gst_wayland_sink_init (GstWaylandSink * self)
   self->force_aspect_ratio = TRUE;
 }
 
+/* must be called with the OBJECT_LOCK */
 static void
 gst_wayland_sink_set_fullscreen (GstWaylandSink * self, gboolean fullscreen)
 {

subprojects/gst-plugins-bad/ext/wayland/gstwaylandsink.h

@@ -60,6 +60,8 @@ struct _GstWaylandSink
 
   gchar *display_name;
 
+  /* If both OBJECT_LOCK and render_lock are needed,
+   * OBJECT_LOCK must be taken first */
   GMutex render_lock;
   GstBuffer *last_buffer;