share/man/man7/simd.7: document simd-enhanced memccpy, strncat

Tested by: developers@, exp-run
Approved by: mjg
MFC after: 1 month
MFC to: stable/14
PR: 275785
Differential Revision: HTTPS://reviews.freebsd.org/D42902

(cherry picked from commit 5fa0fbf40b110e4699032b97a9f540db80997812)