doc: improve dns module's documentation

Make the difference between dns.lookup and other functions even clearer.
I agree. I knew  there were two functions, one calling getaddrinfo in the thread pool, the other doing direct async dns via c-ares, and last time I used the dns module directly I had to spend a few minutes sifting through the documentation to figure out which of `lookup()` and `resolve()` was which... I kindof lament that the one that calls getaddrinfo isn't `dns.getaddrinfo()`.