validate: Minor documentation fixes

https://bugzilla.gnome.org/show_bug.cgi?id=743994

validate/gst/validate/gst-validate-scenario.h

@@ -132,6 +132,14 @@ typedef enum
     GST_VALIDATE_ACTION_TYPE_NO_EXECUTION_NOT_FATAL = 1 << 6,
 } GstValidateActionTypeFlags;
 
+/**
+ * @name: The name of the new action type to add
+ * @implementer_namespace: The namespace of the implementer of the action type
+ * @execute: (virtual do_execute): The function to be called to execute the action
+ * @parameters: (allow-none) (array zero-terminated=1) (element-type GstValidate.ActionParameter): The #GstValidateActionParameter usable as parameter of the type
+ * @description: A description of the new type
+ * @flags: The flags of the action type
+ */
 struct _GstValidateActionType
 {
   GstMiniObject          mini_object;
@@ -146,6 +154,7 @@ struct _GstValidateActionType
   gchar *description;
   GstValidateActionTypeFlags flags;
 
+  /*< private >*/
   gpointer _gst_reserved[GST_PADDING_LARGE];
 };