doc: Add some missing docstrings

gst/gstcapsfeatures.c

@@ -118,6 +118,13 @@ _priv_gst_caps_features_cleanup (void)
   _gst_caps_features_memory_system_memory = NULL;
 }
 
+/**
+ * gst_is_caps_features:
+ *
+ * Checks if @obj is a #GstCapsFeatures
+ *
+ * Returns: %TRUE if @obj is a #GstCapsFeatures %FALSE otherwise
+ */
 gboolean
 gst_is_caps_features (gconstpointer obj)
 {

gst/gstelement.h

@@ -1016,6 +1016,13 @@ GST_API
 void                    gst_element_lost_state          (GstElement * element);
 
 
+/**
+ * GstElementCallAsyncFunc:
+ * @element: The #GstElement this function has been called against
+ * @user_data: Data passed in the function where that callback has been passed
+ *
+ * Callback prototype used in #gst_element_call_async
+ */
 typedef void          (*GstElementCallAsyncFunc)        (GstElement * element,
                                                          gpointer     user_data);
 GST_API

libs/gst/controller/gsttimedvaluecontrolsource.c

@@ -81,6 +81,14 @@ gst_control_point_free (GstControlPoint * cp)
   g_slice_free (GstControlPoint, cp);
 }
 
+/**
+ * gst_control_point_copy:
+ * @cp: The control point to copy
+ *
+ * Copies a #GstControlPoint
+ *
+ * Returns: A copy of @cp
+ */
 GstControlPoint *
 gst_control_point_copy (GstControlPoint * cp)
 {