doc: Add resolveNaptr and naptr rrtype docs

Updates the dns module documentation to include documentation on
the resolveNaptr method, and also adds the option NAPTR to the
list of valid values for rrtype in dns.resolve(hostname[, rrtype],
callback).

PR-URL: https://github.com/nodejs/node/pull/6586
Fixes: https://github.com/nodejs/node/issues/6507


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>