doc: add reference for buffer.inspect()

This commit refers readers to util.inspect() for an explanation
of buffer.inspect().

Fixes: https://github.com/nodejs/node/issues/3918
PR-URL: https://github.com/nodejs/node/pull/3921


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>