doc: add note to util.isBuffer

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

PR-URL: #3790
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/util.markdown

@@ -404,6 +404,8 @@ Returns `true` if the given "object" is a primitive type. `false` otherwise.
 
     Stability: 0 - Deprecated
 
+Use `Buffer.isBuffer()` instead.
+
 Returns `true` if the given "object" is a `Buffer`. `false` otherwise.
 
     var util = require('util');