save-entropy: Add manual page

Succinct manual page for save-entropy(8).

PR: 223998
Reported by: AJ Jordan <alex+freebsd@strugee.net>
Reviewed by: carlavilla@, delphij@, lwhsu@, pauamma_gundo.com
Approved by: delphi, lwhsu (src)
Differential Revision: https://reviews.freebsd.org/D41768