[worker] Document response stream disconnected error (#18567)

* [worker] Document response stream disconnected error

This change documents the `responseStreamDisconnected` error `outcome`
seen in CloudFlare worker logs and error metrics.

Previously, this outcome doesn't appear to be documented, but presents
as an error in metrics and presents as a non-error entry in the
CloudFlare Worker Logs. This change aims to reduce confusion going forward.

Changes introducing `responseStreamDisconnected`: cloudflare/workerd#2503

* Update src/content/docs/workers/observability/errors.mdx

* Moved content under metrics heading

---------

Co-authored-by: ToriLindsay <[email protected]>

Author: jackatbancast

src/content/docs/workers/observability/errors.mdx

@@ -242,6 +242,8 @@ To review whether your application is experiencing any downtime or returning any
 2. In **Account Home**, select **Workers & Pages**.
 3. In **Overview**, select your Worker and review your Worker's metrics.
 
+A `responseStreamDisconnected` event `outcome` occurs when one end of the connection hangs up during the deferred proxying stage of a Worker request flow. This is regarded as an error for request metrics, and presents in logs as a non-error log entry. It commonly appears for longer lived connections such as WebSockets.
+
 ## Debug exceptions
 
 After you have identified your Workers application is returning exceptions, use `wrangler tail` to inspect and fix the exceptions.

src/content/docs/workers/runtime-apis/handlers/tail.mdx

@@ -101,6 +101,7 @@ export default {
     * `exceededMemory`: The Worker invocation exceeded memory limits.
     * `scriptNotFound`: An internal error from difficulty retrieving the Worker script.
     * `canceled`: The worker invocation was canceled before it completed. Commonly because the client disconnected before a response could be sent.
+    * `responseStreamDisconnected`: The response stream was disconnected during deferred proxying. Happens when either the client or server hangs up early.