setgroups.2: Add SECURITY CONSIDERATIONS, fix the groups limit, rework

Add a new SECURITY CONSIDERATIONS section describing in details what the
new behavior is after commit 9da2fe96ff2e ("kern: fix setgroups(2) and
getgroups(2) to match other platforms"), what setgroups(2) does not
do anymore, and how programs using it are affected.

Fix the groups limit after commit 9da2fe96ff2e ("kern: fix setgroups(2)
and getgroups(2) to match other platforms").

Prefer a terminology referring to POSIX terms, i.e., use "effective
group list" instead of "group access list".

While here, fix some style.

Note for MFC to stable/14: The content will have to be revised as the
new behavior is not in place. The latter should still be mentioned as
upcoming in 15.

Fixes: 9da2fe96ff2e ("kern: fix setgroups(2) and getgroups(2) to match other platforms")
MFC after: 5 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52284