Fix information about $firewall_myservices

After r273201 it is supported "/{udp,tcp,proto}" suffix into
$firewall_myservices, and in the rc.conf the information is outdated.

Reviewed by: bcr, rgrimes
Approved by: re (gjb), doc (bcr), src (rgrimes)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D17338