ipfw.8: synopsis misses nat show form

Document the existing behavior, which is currently only available by
reading third party documentation or the source code itself.

PR: 254617
Submitted by: Oliver Kiddle
Differential Revision: https://reviews.freebsd.org/D30189

(cherry picked from commit c8250c5ada85fec8936e8eb8695d7cb80a3ce8ab)