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>
Merge remote-tracking branch 'upstream/v0.10' into v0.10