memcmp.3: Clarify return value

The return value is not required to be the difference between the
differing bytes, only less than zero, zero, or greater than zero.

Reviewed by: fuz
Event: Kitchener-Waterloo Hackathon 202406
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47683

(cherry picked from commit 5cc53d79655bf7f67b1a794c088d896aa6475c2e)