setcred.2: Remove now obsolete mention of setgroups(2)'s different behavior
Reviewed by: gbe
Fixes: 9da2fe96ff2e ("kern: fix setgroups(2) and getgroups(2) to match other platforms")
MFC after: 5 days
MFC to: stable/15
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52285