pf.4: Add pf_enable to SYNOPSIS

MFC after: 3 days
Reviewed by: Pau Amma <pauamma@gundo.com>
Reviewed by: Felix Johnson <felix.the.red@gmail.com>
Differential Revision: https://reviews.freebsd.org/D51117

(cherry picked from commit 65bc8e9192dd19bca3ed89586a23a74cd449f7b2)