siftr: document siftr probe to man page of the tcp provider

This man page documents what is currently implemented in siftr.d.
It doesn't work right now in head, but in stable/13. Follow-up
commits will fix it for head.

Reviewed by: cc, pauamma_gundo.com
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D40809

(cherry picked from commit 0631830a7a3c9625f1678b0b6ab8b525ba5e3873)