gtktypes: Document GtkCssStyleChange

This is used in the css_changed virtual function of GtkWidget.

gtk/gtktypes.h

@@ -37,6 +37,12 @@ typedef struct _GtkATContext           GtkATContext;
 typedef struct _GtkBitset              GtkBitset;
 typedef struct _GtkBuilder             GtkBuilder;
 typedef struct _GtkBuilderScope        GtkBuilderScope;
+
+/**
+ * GtkCssStyleChange:
+ *
+ * A CSS style change.
+ */
 typedef struct _GtkCssStyleChange      GtkCssStyleChange;
 typedef struct _GtkEventController     GtkEventController;
 typedef struct _GtkGesture             GtkGesture;