cred: Update some comments after 'cr_gid' is no more cr_groups[0]

While here, fix style of some already-updated comments.

Fixes: be1f7435ef218b1d ("kern: start tracking cr_gid outside of cr_groups[]")
MFC after: 5 days
MFC to: stable/15
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52266