setkey.8: Improve direction descriptions

Be more precise in the definition of policy directions
and policy levels.

PR: 250177
Reported by: Bram Ton <bram at cbbg dot nl>
Reviewed by: gbe, ae
Differential Revision: https://reviews.freebsd.org/D26719

(cherry picked from commit 06bfd0b9143b0409b1b343976ca1d6f41f796423)