doc: add description of autoAllocateChunkSize in ReadableStream

Refs: https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream/ReadableStream PR-URL: #48004 Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com> Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: debadree25

doc/api/webstreams.md

@@ -124,7 +124,10 @@ added: v16.5.0
  * Returns: A promise fulfilled with `undefined`.
  * `type` {string} Must be `'bytes'` or `undefined`.
  * `autoAllocateChunkSize` {number} Used only when `type` is equal to
- `'bytes'`.
+ `'bytes'`. When set to a non-zero value a view buffer is automatically
+ allocated to `ReadableByteStreamController.byobRequest`. When not set
+ one must use stream's internal queues to transfer data via the default
+ reader `ReadableStreamDefaultReader`.
 * `strategy` {Object}
  * `highWaterMark` {number} The maximum internal queue size before backpressure
  is applied.