mac80211: Update beacon_update callback documentation

Index: wireless-2.6/include/net/mac80211.h
===================================================================
@@ -890,7 +890,14 @@ struct ieee80211_ops {
 
 	/* Setup beacon data for IBSS beacons. Unlike access point (Master),
 	 * IBSS uses a fixed beacon frame which is configured using this
-	 * function. This handler is required only for IBSS mode. */
+	 * function.
+	 * If the driver returns success (0) from this callback, it own
+	 * the skb. That means the driver is responsible to kfree_skb() it.
+	 * The control structure is not dynamically allocated. That means the
+	 * driver does not own the pointer and if it needs it somewhere
+	 * outside of the context of this function, it must copy it
+	 * somewhere else.
+	 * This handler is required only for IBSS mode. */
 	int (*beacon_update)(struct ieee80211_hw *hw,
 			     struct sk_buff *skb,
 			     struct ieee80211_tx_control *control);