share/man/man7/simd.7: document strrchr scalar, baseline implementation

Also mention missing rindex() entry, which is provided through
strrchr().

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/D42217