pf.conf.5: use subsections and reword titles

The current manpage has two PARAMETERS top-level sections, which can be
confusing when searching or browsing documentation quickly.

Since the parameters described in these two sections are actually
applicable within the scope of the sections they follow (ETHERNET
FILTERING and PACKET FILTERING respectively), I propose we move them to
subsections.

I also propose we reword the title of one of those subsections to
facilitate quick searching or browsing.

MFC after: 3 days
Reviewed by: imp, ziaee
Pull Request: https://github.com/freebsd/freebsd-src/pull/1516
Closes: https://github.com/freebsd/freebsd-src/pull/1516