better Doxygen documentation of icon()

server/drivers/CFontz633.c

@@ -845,7 +845,8 @@ CFontz633_set_char (Driver *drvthis, int n, unsigned char *dat)
  * \param x        Horizontal character position (column).
  * \param y        Vertical character position (row).
  * \param icon     synbolic value representing the icon.
- * \return  Information whether the icon is handled here or needs to be handled by the server core.
+ * \retval 0       Icon has been successfully defined/written.
+ * \retval <0      Server core shall define/write the icon.
  */
 MODULE_EXPORT int
 CFontz633_icon (Driver *drvthis, int x, int y, int icon)