doc: add note to util.isBuffer

Since util.isBuffer is deprecated, we should be explicit that
Buffer.isBuffer should be used instead.

PR-URL: https://github.com/nodejs/node/pull/3790


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>