share/man/man7/simd.7: document SIMD-enhanced strcat

Sponsored by: The FreeBSD Foundation
Tested by: developers@, exp-run
Approved by: mjg
MFC after: 1 month
MFC to: stable/14
PR: 275785
Differential Revision: https://reviews.freebsd.org/D42600

(cherry picked from commit c983825a77f276ab71a34baee0250d55a84f59fd)