share/man/man7/simd.7: document amd64/strchrnul SIMD implementaionts

While we are at it, remove mention of x86-64-v3 and v4 variants of
strlen(3) which were omitted when D40693 landed.

Sponsored by: The FreeBSD Foundation
Approved by: mjg
Differential Revision: https://reviews.freebsd.org/D41333