chroot.2: chroot() is not intended for security; document attack

chroot.2: chroot() is not intended for security; document attack

It is unfortunate that this discourages this use of chroot(2)
without pointing out alternative solutions - for example,
OpenSSH and vsftpd both still rely on chroot(2) for security.

Bind mounts should theoretically be usable as a replacement, but
currently, they have a similar problem (CVE-2015-2925) that hasn't
been fixed in ~6 months, so I'd rather not add it to the manpage
as a solution before a fix lands.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>