users/config: be more precise about trafficClass value

Although described, it was not obvious to the author
what exact value needs to be specified. To improve this,
it was explicitly stated that it's about the respective
IPv4 and IPv6 header fields.

Additionally, users most likely want to specify a DSCP
value. To help in this case the required bit shift was
mentioned.

users/config.rst

@@ -1221,7 +1221,10 @@ The ``options`` element contains all other global configuration options.
 
 .. option:: options.trafficClass
 
-    Specify a type of service (TOS)/traffic class of outgoing packets.
+    Specify an IPv4 type of service (TOS)/IPv6 traffic class for outgoing
+    packets. To specify a differentiated services code point (DSCP) the value
+    must be bit shifted to the left by two to take the two least significant
+    ECN bits into account.
 
 .. option:: options.stunServer
     :aliases: options.stunServers