doc: reorder stream 'readable' paragraphs

Fixes: https://github.com/nodejs/node/issues/40136

PR-URL: https://github.com/nodejs/node/pull/40212


Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>