docs: user/site: improve explanation of basic_rates / supported_rates (#1540)

docs/user/site.rst

@@ -134,8 +134,15 @@ wifi24 \: optional
     Additionally it is possible to configure the ``supported_rates`` and ``basic_rate``
     of each radio. Both are optional, by default hostapd/driver dictate the rates.
     If ``supported_rates`` is set, ``basic_rate`` is required, because ``basic_rate``
-    has to be a subset of ``supported_rates``.
-    The example below disables 802.11b rates.
+    has to be a subset of ``supported_rates``. Possible values are: 
+
+    - 6000, 9000, 12000, 18000, 24000, 36000, 48000, 54000 (OFDM)
+    - 1000, 5500, 11000 (legacy 802.11b, DSSS)
+
+    The example below disables legacy 802.11b rates (DSSS) for performance reasons.  
+    For backwards compatibility in 802.11, this setting only effects 802.11a/b/g rates. 
+    I.e in 802.11n 6 MBit/s is announced  all time. In 802.11ac the field is used to 
+    derive the length of a packet.
 
     ``ap`` requires a single parameter, a string, named ``ssid`` which sets the
     interface's ESSID. This is the WiFi the clients connect to.