doc: add note to util.isBuffer
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 <f1782d15e31ad502ea2c097c21cc235811d0cb7c@rocketmail.com>
Reviewed-By: Colin Ihrig <838f779c91673db29b8572ea3f54d191d3878ae2@gmail.com>
Reviewed-By: James M Snell <2d64a127ffc94dbe6295928a16bedf8b26814159@gmail.com>

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node