doc: clarify API buffer.concat
doc: clarify API buffer.concat

* Add a simple example for buffer.concat
* Change grammar slightly.

Fixes: #3219
Reviewed-By: James M Snell <2d64a127ffc94dbe6295928a16bedf8b26814159@gmail.com>
Reviewed-By: Trevor Norris <9cb5491eb5770dd8e0bf77a8a0335abf5f0488c5@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/3255

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