doc: Undocumented NAPTR DNS record type

[MrQubo]

The DNS documentation is missing resolveNaptr().
'NAPTR' should be added as valid value for rrtype below dns.resolve(hostname[, rrtype], callback).

DNS documentation: https://nodejs.org/dist/latest-v6.x/docs/api/dns.html

[cjihrig]

Documentation PRs are welcome :-)