cred: 'struct ucred': Rework comment on 'cr_gid'/cr_groups[]

Make it more straightforward for consumers. Also, recommend using
cr_sgroups[] for supplementary groups, as this can generally alleviate
index problems and will help us spot and change consumers if/when
'struct ucred' is evolved incompatibly (and cr_sgroups[] suppressed).

MFC after: 5 days
MFC to: stable/15
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52269

(cherry picked from commit 0abeb8d8d8df1842b0c2d2276c958b0150579a1c)