docs: add field descriptions for GtkSettingsValue

gtk/gtksettings.h

@@ -63,6 +63,13 @@ struct _GtkSettingsClass
   void (*_gtk_reserved4) (void);
 };
 
+/**
+ * GtkSettingsValue:
+ * @origin: Origin should be something like "filename:linenumber" for
+ *    rc files, or e.g. "XProperty" for other sources.
+ * @value: Valid types are LONG, DOUBLE and STRING corresponding to
+ *    the token parsed, or a GSTRING holding an unparsed statement
+ */
 struct _GtkSettingsValue
 {
   /* origin should be something like "filename:linenumber" for rc files,