setgroups.2: Add SECURITY CONSIDERATIONS, rework

Add a new SECURITY CONSIDERATIONS section contrasting the current
behavior with the new one in force starting from FreeBSD 15.

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

While here, fix some style.

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

(cherry picked from commit 6d22cd6b5f8b5604f1fe9e70930b1506f990e31e)

As indicated in the original commit message, the manual page was
specifically modified as stable/14's setgroups(2) still
has the old behavior.

The original commit message above was reworked to reflect the actual
commit content.