doc: make sure that calls to .read() are looped

The 'readable' event assumes that calls to readable.read() happens
within that event handler until readable.read() returns null.

Fixes: https://github.com/nodejs/node/issues/20503
PR-URL: https://github.com/nodejs/node/pull/25375


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>