pf.conf.5: Document new 'match' keyword

Reviewed by: bcr
Obtained from: pfSense
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31117

(cherry picked from commit 0351b9875a3eb5eb4b502a4106b1917ce954e959)