doc: clarify explanation of first stream section

The last sentence of the explanation for the first stream section
seemed a bit confusing. I tried to change the sentence to clarify it.

Additionally, the sections were turned into a numbered list to be more
clear about which section is being described, and improve readability.

PR-URL: https://github.com/nodejs/node/pull/4234


Reviewed-By: James M Snell <jasnell@gmail.com>