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
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D40809