pf.conf.5: rephrase macro section

In the manual pages for configuration files based on parse.y, describe
the syntax of both defining and using macros, rather than exclusively
relying on examples, which some of the pages do not even provide.
In those pages containing tables of content, also clarify that
the "Macros" section contains *definitions* of variables.
Both changes were already committed to vm.conf(5) earlier.

OK florian@, and deraadt also requests keeping these manuals in sync.

Obtained from: OpenBSD, schwarze <schwarze@openbsd.org>, 20aba01e82
Sponsored by: Rubicon Communications, LLC ("Netgate")