docs: nwfilter: add missing dscp attribute

docs: nwfilter: add missing dscp attribute

Added attribute dscp to below supported protocols table in nwfilter docs.
IPV4 (ip)
TCP/UDP/SCTP (tcp/udp/sctp)
ICMP (icmp)
IGMP,ESP,AH,UDPLITE,'ALL' (igmp,esp,ah,udplite,all)
TCP/UDP/SCTP over IPV6 (tcp-ipv6,udp-ipv6,sctp-ipv6)
ICMPV6 (icmpv6)
IGMP,ESP,AH,UDPLITE,'ALL' over IPv6 (igmp-ipv6,esp-ipv6,ah-ipv6,udplite-ipv6,all-ipv6)

Here is a simple example:
[root@localhost ~]# virsh nwfilter-dumpxml myself
<filter name='myself' chain='root'>
  <uuid>7192ef51-cd50-4f14-ad7b-fa5c69ea19e3</uuid>
  <rule action='accept' direction='in' priority='500'>
    <ip dscp='1'/>
  </rule>
  <rule action='accept' direction='in' priority='500'>
    <tcp dscp='1'/>
  </rule>
  <rule action='accept' direction='in' priority='500'>
    <tcp-ipv6 dscp='2'/>
  </rule>
  <rule action='accept' direction='in' priority='500'>
    <icmp dscp='55'/>
  </rule>
  <rule action='accept' direction='in' priority='500'>
    <icmpv6 dscp='55'/>
  </rule>
  <rule action='accept' direction='in' priority='500'>
    <udp dscp='3'/>
  </rule>
  <rule action='accept' direction='in' priority='500'>
    <udp-ipv6 dscp='4'/>
  </rule>
  <rule action='accept' direction='in' priority='500'>
    <udplite dscp='5'/>
  </rule>
  <rule action='accept' direction='in' priority='500'>
    <udplite-ipv6 dscp='6'/>
  </rule>
  <rule action='accept' direction='in' priority='500'>
    <esp dscp='7'/>
  </rule>
  <rule action='accept' direction='in' priority='500'>
    <esp-ipv6 dscp='8'/>
  </rule>
  <rule action='accept' direction='in' priority='500'>
    <ah dscp='9'/>
  </rule>
  <rule action='accept' direction='in' priority='500'>
    <ah-ipv6 dscp='10'/>
  </rule>
  <rule action='accept' direction='in' priority='500'>
    <sctp dscp='11'/>
  </rule>
  <rule action='accept' direction='in' priority='500'>
    <sctp-ipv6 dscp='55'/>
  </rule>
  <rule action='accept' direction='in' priority='500'>
    <igmp dscp='55'/>
  </rule>
  <rule action='accept' direction='in' priority='500'>
    <all-ipv6 dscp='55'/>
  </rule>
  <rule action='accept' direction='in' priority='500'>
    <all dscp='55'/>
  </rule>
</filter>

Signed-off-by: Ján Tomko <jtomko@redhat.com>