jail.8: Update the man page for allow.nfsd

Commit bba7a2e89602 added "allow.nfsd" to optionally allow
mountd/nfsd to be run inside a vnet prison when the kernel
is built with "options VNET_NFSD".

This patch updates the man page for this change.

This is a content change.

Reviewed by: jamie, bcr (manpages)
MFC after: 4 months
Differential Revision: https://reviews.freebsd.org/D37665