doc: edit Stream api grammar

Sections:
  - Introduction
  - Orginization of this Document
  - Object Mode
  - Buffering
  - API for Stream Implementers

PR-URL: https://github.com/nodejs/node/pull/9100


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>