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