docs: add note/warning to gst_index_get_writer_id() docs about the OBJECT_LOCK

https://bugzilla.gnome.org/show_bug.cgi?id=646811

gst/gstindex.c

@@ -695,6 +695,12 @@ gst_index_gtype_resolver (GstIndex * index, GstObject * writer,
  * to a string. That string will be used to register or look up an id
  * in the index.
  *
+ * <note>
+ * The caller must not hold @writer's #GST_OBJECT_LOCK, as the default
+ * resolver may call functions that take the object lock as well, and
+ * the lock is not recursive.
+ * </note>
+ *
  * Returns: TRUE if the writer would be mapped to an id.
  */
 gboolean