pf: Update documentation regarding matching, scrubbing and reassembly

Update pf documentation:

• default behaviour of fragment reassembly
• introduction of scrub option for filter rules
• disadvantages of using the old scrub ruleset
• options supported for match rules
• fix missing list block end
• remove duplicate description of match filter rule
• update example to modern syntax

Reviewed by: kp
Fragments obtained from: OpenBSD
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D42270