doc: clarify dns.lookup vs dns.resolve

Clarify and emphasize the differences between dns.lookup and the rest of
the functions in the dns module.

PR-URL: https://github.com/joyent/node/pull/8726


Reviewed-by: Chris Dickinson <christopher.s.dickinson@gmail.com>

Cherry-picked-from: https://github.com/joyent/node/commit/5ff59453a483a43f1563504989f8e2d0ec253de1