protect.1: Document that protect(1) does not work in jails

The reason is that in order to protect a process procctl(2) needs
the PRIV_VM_MADV_PROTECT privilege, which is currently denied in jails
(see kern_jail.c).

MFC after: 1 week

(cherry picked from commit 6452fb1e87ed9d00b52fa1e63e7c3a7516c9586c)