Add some notes on which channels can be used with HT40- and HT40+

This is not a complete list, but at least it can provide some help for
whoever is trying to configure hostapd for 802.11n HT40 operation.

hostapd/hostapd.conf

@@ -384,6 +384,14 @@ wme_ac_vo_acm=0
 #	channel below the primary channel; [HT40+] = both 20 MHz and 40 MHz
 #	with secondary channel below the primary channel
 #	(20 MHz only if neither is set)
+#	Note: There are limits on which channels can be used with HT40- and
+#	HT40+. Following table shows the channels that may be available for
+#	HT40- and HT40+ use per IEEE 802.11n Annex J:
+#	freq		HT40-		HT40+
+#	2.4 GHz		5-13		1-7 (1-9 in Europe/Japan)
+#	5 GHz		40,48,56,64	36,44,52,60
+#	(depending on the location, not all of these channels may be available
+#	for use)
 # Spatial Multiplexing (SM) Power Save: [SMPS-STATIC] or [SMPS-DYNAMIC]
 #	(SMPS disabled if neither is set)
 # HT-greenfield: [GF] (disabled if not set)