[worker] Document response stream disconnected error

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

@@ -231,6 +231,10 @@ Runtime errors will occur within the runtime, do not throw up an error page, and
 | `Memory limit`<br/>`would be exceeded`<br/> `before EOF` | Trying to read a stream or buffer that would take you over the [memory limit](/workers/platform/limits/#memory). |
 | `daemonDown`                                             | A temporary problem invoking the Worker.                                                                         |
 
+### Response Stream Disconnected
+
+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. Commonly appears for longer lived connections such as websockets.
+
 ## Identify errors: Workers Metrics
 
 To review whether your application is experiencing any downtime or returning any errors:

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.