pfctl.8: Explicitly mention that tables created by -T{add,replace} are persistent

pf.conf(5)'s TABLE section explains it, but lets be clear about it in the
table command descriptions, too.

"go for it" jmc

Obtained from: OpenBSD, kn <kn@openbsd.org>, 8d4231551f
Sponsored by: Rubicon Communications, LLC ("Netgate")