doc: use Buffer.from() instead of new Buffer()

Use new API of Buffer to developers in most documents.

PR-URL: https://github.com/nodejs/node/pull/6367


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>