doc,stream: remove wrong remark on readable.read

The returned chunk is *never* longer than `size`.

PR-URL: https://github.com/nodejs/node/pull/15014


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>