NTP pools {0,1,3}.freebsd.pool.ntp.org contain only NTP servers
wtth IPv4 addresses only. Whereas the NTP pool 2.freebsd.pool.ntp.org
contains servers with IPv4 and IPv6 addresses. Use it instead.
This allows FreeBSD installed at IPv6-only sites to work out
of the box.
While here, document all four pools, allowing the user to uncomment
one of the other NTP pools when desired.
See PR/264088.