doc: improve dns module's documentation

Make the difference between dns.lookup and other functions even clearer.
You mention only upsides, and no downsides, which could/has lead to people asking, "why isn't dns.resolve the default lookup mechanism in node?".

getaddrinfo might be faster if the systems does local cacheing. getaddrinfo may be able to resolve addresses, like `.loca`l network addresses available only using mDNS, that can't be resolved at all using DNS. And I haven't tried this, but I suspect that dns.resolve also won't qualify names, so attempts to lookup names like `'mail'` which might normally resolve to `'mail.yourcompany.com'` will fail. These failures to sucessfully lookup names that can be found by every other application on the user's system can be a source of frustration. dns.resolve() is best reserved for bulk, parallel lookup of names known to be fully qualified and available via DNS, I think.