doc: fix two doc errors in stream and process

`process.stdout` always blocks as of
20176a98

`WritableState.buffer` is `getBuffer()` as of
91586661

PR-URL: https://github.com/nodejs/node/pull/2549


Reviewed-By: Alexis Campailla <orangemocha@nodejs.org>
Reviewed-By: Evan Lucas <evanlucas@me.com>