I dug around some old mailing lists. The warning pertains to cases where a person has a normal account on the host, and root access in a jail. He can then suid a binary, eg. vi, and use that suid executable on the host.
If you where to have this edge case, the problem is easy to fix, just chmod 0700 your jail dir on the host.
SHM was also mentionen but as I stated, it's disabled by default.