doc: improve dns module's documentation

Make the difference between dns.lookup and other functions even clearer.

PR-URL: https://github.com/joyent/node/pull/8747


Signed-off-by: Timothy J Fontaine <tjfontaine@gmail.com>

Cherry-picked-from: https://github.com/joyent/node/commit/542234ad984fd1fd90958f07f7f122530e7fb9c6