doc: add note to readable stream async iterator

PR-URL: #19331
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/stream.md

@@ -1208,7 +1208,10 @@ print(fs.createReadStream('file')).catch(console.log);
 
 If the loop terminates with a `break` or a `throw`, the stream will be
 destroyed. In other terms, iterating over a stream will consume the stream
-fully.
+fully. The stream will be read in chunks of size equal to the `highWaterMark`
+option. In the code example above, data will be in a single chunk if the file
+has less then 64kb of data because no `highWaterMark` option is provided to
+[`fs.createReadStream()`][].
 
 ### Duplex and Transform Streams