Add a note on using 'iw list' to determine multi-BSS support

This adds a pointer to hostapd.conf to using 'iw list' with the current
nl80211 drivers to determine supported interface combinations.

Signed-off-by: Jouni Malinen <j@w1.fi>

hostapd/hostapd.conf

@@ -1731,6 +1731,11 @@ own_ip_addr=127.0.0.1
 # - is not the same as the MAC address of the radio
 # - is not the same as any other explicitly specified BSSID
 #
+# Not all drivers support multiple BSSes. The exact mechanism for determining
+# the driver capabilities is driver specific. With the current (i.e., a recent
+# kernel) drivers using nl80211, this information can be checked with "iw list"
+# (search for "valid interface combinations").
+#
 # Please note that hostapd uses some of the values configured for the first BSS
 # as the defaults for the following BSSes. However, it is recommended that all
 # BSSes include explicit configuration of all relevant configuration items.