pf.conf.5: Markup optional ICMP/ICMP6 codes as such

Only icmp(4)/icmp6(4) types are required for icmp-type'/icmp6-type' rules
while codes are optional.

From Martin Vahlensieck < openbsd at academicsolutions dot ch >, thanks!

Obtained from: OpenBSD, kn <kn@openbsd.org>, 5171218709
Sponsored by: Rubicon Communications, LLC ("Netgate")