perlfunc: Should chdir("/") immed after chroot

Inspired by Coverity perl5 CID 28931: Insecure chroot: (CHROOT)
chroot_call.

The Perl chroot() is a thin wrapper around the system call, so the
chdir("/") should not go there.  But adding a note about the chdir()
being a good idea to perlfunc/chroot.

The committer reworded the inserted text for grammar and readability,
and the commit message to conform to git standards, and is pushing the
commit with the concurrence of the author.