cookbooks/deve/pseudovariables: list cseqno field for $uac_req(...)

docs/cookbooks/devel/pseudovariables.md

@@ -2290,6 +2290,7 @@ The key can be:
 - sock - local socket to be used for sending (proto:address:port)
 - callid - SIP-Call-ID of the generated request (by default, a call-id
     is generated)
+- cseqno - CSeq number to be used if greater than 0
 - all - alias useful to reset all fields - $uac_req(all) = $null;
 - evroute - it has to be set to 1 in order to execute
     event_route\[uac:reply\] when reply is received