doc: Add release notes for #18335 (rpc work queue exceeded error)

doc/release-notes.md

@@ -105,6 +105,10 @@ Low-level changes
 
 RPC
 ---
+- The RPC server can process a limited number of simultaneous RPC requests.
+  Previously, if this limit was exceeded, `bitcoind` would respond with
+  [status code 500 (`HTTP_INTERNAL_SERVER_ERROR`)](https://en.wikipedia.org/wiki/List_of_HTTP_status_codes#5xx_server_errors).
+  Now it returns status code 503 (`HTTP_SERVICE_UNAVAILABLE`). (#18335)
 
 Tests
 -----