share/man/man7/simd.7: document simd-enhanced memchr(3), strnlen(3)
Sponsored by: The FreeBSD Foundation
Approved by: mjg
MFC after: 1 week
MFC to: stable/14
Differential Revision: https://reviews.freebsd.org/D41598
(cherry picked from commit 2a4096b012449d09e9bc3e912f762e4786092a3c)