doc: dns.resolve fix callback argument description

The dns.resolve documentation stated that an array of IP
addresses would be returned in the callback. This is true
for everything other than the SOA record which returns an object.
This fixes that documentation.

Fixes: https://github.com/nodejs/node/issues/6506
PR-URL: https://github.com/nodejs/node/pull/7532


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Roman Reiss <me@silverwind.io>