Add some gtk-doc comments.

src/backends/meta-pointer-constraint.h

@@ -35,6 +35,11 @@ G_BEGIN_DECLS
 G_DECLARE_DERIVABLE_TYPE (MetaPointerConstraint, meta_pointer_constraint,
                           META, POINTER_CONSTRAINT, GObject);
 
+/**
+ * MetaPointerConstraintClass:
+ * @constrain: the virtual function pointer for
+ *             meta_pointer_constraint_constrain().
+ */
 struct _MetaPointerConstraintClass
 {
   GObjectClass parent_class;