cred: 'kern.ngroups' tunable: Rework the comment on the initial value

Remove the parts about the effective GID being stored as the first
element of the groups array, as this is generally not the case now after
commit be1f7435ef21 ("kern: start tracking cr_gid outside of
cr_groups[]") (some credentials export structures still have the old
layout, and need to do the 'ngroups_max + 1' computation, but given the
computation needed to obtain a size in bytes, this detail is not really
important here).

Remove the part about 32-bit machines, avoiding another modification
when support for them is removed.

No functional change.

MFC after: 5 days
MFC to: stable/15
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D52265

(cherry picked from commit a61e7b5c98f840b4a7777d718218d0f63569c52a)