pf.conf.5: clarify filter evaluation and anchor loading

• Clarify that filter rules are evaluated once per packet and interface,

not only once per packet.

• Clarify that the syntax anchor "name" { ... } both loads and

evaluates the anchor, rather than merely loading it.
Triggered by questions from Benedikt Neuffer <bene at usta dot de>.
OK mikeb@

Obtained from: OpenBSD, schwarze <schwarze@openbsd.org>, 7528bd0ba2
Sponsored by: Rubicon Communications, LLC ("Netgate")