Add more hostapd.conf documentation for hw_mode with HT/VHT

Try to make it more obvious that hw_mode=a needs to be used with HT and
VHT when using the 5 GHz band.

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

hostapd/hostapd.conf

@@ -125,11 +125,13 @@ ssid=test
 # ieee80211d=1 and local_pwr_constraint configured.
 #spectrum_mgmt_required=1
 
-# Operation mode (a = IEEE 802.11a, b = IEEE 802.11b, g = IEEE 802.11g,
-# ad = IEEE 802.11ad (60 GHz); a/g options are used with IEEE 802.11n, too, to
-# specify band). When using ACS (see channel parameter), a special value "any"
-# can be used to indicate that any support band can be used. This special case
-# is currently supported only with drivers with which offloaded ACS is used.
+# Operation mode (a = IEEE 802.11a (5 GHz), b = IEEE 802.11b (2.4 GHz),
+# g = IEEE 802.11g (2.4 GHz), ad = IEEE 802.11ad (60 GHz); a/g options are used
+# with IEEE 802.11n (HT), too, to specify band). For IEEE 802.11ac (VHT), this
+# needs to be set to hw_mode=a. When using ACS (see channel parameter), a
+# special value "any" can be used to indicate that any support band can be used.
+# This special case is currently supported only with drivers with which
+# offloaded ACS is used.
 # Default: IEEE 802.11b
 hw_mode=g
 
@@ -477,6 +479,7 @@ wmm_ac_vo_acm=0
 # 0 = disabled (default)
 # 1 = enabled
 # Note: You will also need to enable WMM for full HT functionality.
+# Note: hw_mode=g (2.4 GHz) and hw_mode=a (5 GHz) is used to specify the band.
 #ieee80211n=1
 
 # ht_capab: HT capabilities (list of flags)
@@ -530,6 +533,7 @@ wmm_ac_vo_acm=0
 # 0 = disabled (default)
 # 1 = enabled
 # Note: You will also need to enable WMM for full VHT functionality.
+# Note: hw_mode=a is used to specify that 5 GHz band is used with VHT.
 #ieee80211ac=1
 
 # vht_capab: VHT capabilities (list of flags)