Document MoQ known issue: extra subgroup header field (cloudflare#25132)

* MoQ known issue: extra subgroup header field

Document inadvertent divergence from draft-ietf-moq-transport-07

* Update index.mdx

minor style updates

Author: englishm-cloudflare

src/content/docs/moq/index.mdx

@@ -24,3 +24,33 @@ For the most up-to-date documentation on the protocol, please visit the IETF wor
   Safari does not yet have fully functional WebTransport support. Apple never publicly commits to timelines for new features like this. However, Apple has indicated their [intent to support WebTransport](https://github.com/WebKit/standards-positions/issues/18#issuecomment-1495890122). An Apple employee is even a co-author of the [WebTransport over HTTP/3](https://datatracker.ietf.org/doc/draft-ietf-webtrans-http3/) draft. Since Safari 18.4 (2025-03-31), an early (not yet fully functional) implementation of the WebTransport API has been available for testing behind a developer-mode / advanced settings feature flag (including on iOS).
 
   Until Safari has a fully functional WebTransport implementation, some MoQ use cases may require a fallback to WebRTC, or, in some cases, WebSockets.
+
+## Known Issues
+
+* Extra Subgroup header field
+
+  The current implementation includes a `subscribe_id` field in Subgroup Headers which [`draft-ietf-moq-transport-07`](https://datatracker.ietf.org/doc/html/draft-ietf-moq-transport-07) omits. 
+  
+  In section 7.3.1, `draft-ietf-moq-transport-07` [specifies](https://www.ietf.org/archive/id/draft-ietf-moq-transport-07.html#section-7.3.1):
+  
+  ```txt
+  STREAM_HEADER_SUBGROUP Message {
+    Track Alias (i),
+    Group ID (i),
+    Subgroup ID (i),
+    Publisher Priority (8),
+  }
+  ```
+  
+  Whereas our implementation expects and produces:
+  ```txt
+  STREAM_HEADER_SUBGROUP Message {
+    Subscribe ID (i),
+    Track Alias (i),
+    Group ID (i),
+    Subgroup ID (i),
+    Publisher Priority (8),
+  }
+  ```
+  
+  This was erroroneously left over from a previous draft version and will be fixed in a future release. Thank you to [@yuki-uchida](https://github.com/yuki-uchida) for reporting.