doc: update writable.write return value

stream.md is updated to explain the return value of
writable.write(chunk) precisely.

PR-URL: https://github.com/nodejs/node/pull/9468
Fixes: https://github.com/nodejs/node/issues/9247


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ron Korving <ron@ronkorving.nl>
Reviewed-By: Roman Reiss <me@silverwind.io>