ucred.9: crsetgroups_and_egid(): Clarify differences with crsetgroups()

MFC after: 5 days
MFC to: stable/15
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52288

(cherry picked from commit 6b26d0a335b6222744e2778753086e16389b6029)