miext: damage: document DamageScreenFuncsRec

This struct (and associated functions) needs to be part of public driver ABI,
so video drivers can get notifications on damage certain operations, but
there hasn't been any documentation on it.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

miext/damage/damage.h

@@ -46,6 +46,18 @@ typedef void (*DamageScreenRegisterFunc) (DrawablePtr, DamagePtr);
 typedef void (*DamageScreenUnregisterFunc) (DrawablePtr, DamagePtr);
 typedef void (*DamageScreenDestroyFunc) (DamagePtr);
 
+/* @public
+ *
+ * @brief Driver callbacks for getting notified on several damage calls
+ *
+ * The pointer to this struct can be obtained via DamageGetScreenFuncs().
+ * Drivers can inject themselves here, in order to get notified on
+ * DamageCreate(), DamageRegister(), DamageUnregister(), DamageDestroy().
+ *
+ * This should ONLY be touched by video drivers, nobody else.
+ *
+ * So far the only one using it is the proprietary NVidia driver.
+ */
 typedef struct _damageScreenFuncs {
     DamageScreenCreateFunc Create;
     DamageScreenRegisterFunc Register;