doc: fix process.stdin example

Fixes: https://github.com/nodejs/node/issues/20503

PR-URL: https://github.com/nodejs/node/pull/25344
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/process.md

@@ -1856,8 +1856,9 @@ a [Readable][] stream.
 process.stdin.setEncoding('utf8');
 
 process.stdin.on('readable', () => {
-  const chunk = process.stdin.read();
-  if (chunk !== null) {
+  let chunk;
+  // Use a loop to make sure we read all available data.
+  while ((chunk = process.stdin.read()) !== null) {
     process.stdout.write(`data: ${chunk}`);
   }
 });