chore(doc/fetch) update doc for fetch duplex (#1760) (#1765)

* chore(doc/fetch) update doc for fetch duplex (#1760)

* chore(doc/fetch) update doc for fetch duplex (#1760)

README.md

@@ -230,9 +230,15 @@ const data = {
   },
 }
 
-await fetch('https://example.com', { body: data, method: 'POST' })
+await fetch('https://example.com', { body: data, method: 'POST', duplex: 'half' })
 ```
 
+#### `request.duplex`
+
+- half
+
+In this implementation of fetch, `request.duplex` must be set if `request.body` is `ReadableStream` or `Async Iterables`. And fetch requests are currently always be full duplex. More detail refer to [Fetch Standard.](https://fetch.spec.whatwg.org/#dom-requestinit-duplex)
+
 #### `response.body`
 
 Nodejs has two kinds of streams: [web streams](https://nodejs.org/dist/latest-v16.x/docs/api/webstreams.html), which follow the API of the WHATWG web standard found in browsers, and an older Node-specific [streams API](https://nodejs.org/api/stream.html). `response.body` returns a readable web stream. If you would prefer to work with a Node stream you can convert a web stream using `.fromWeb()`.