Merge #21484: doc: Add release notes for #18335

51eef4a doc: Add release notes for #18335 (rpc work queue exceeded error) (Larry Ruane)

Pull request description:

  Follow-on to #18335

ACKs for top commit:
  darosior:
    ACK 51eef4a

Tree-SHA512: 863d92cb1c23493d9c8c42ed89b30ebd59092e44f159de4cefbabfe4101e7d7d40f24776ff3fcf39dedf90b45fc25845cf7a2177af38729ce2118d75c3cd779b

Author: MarcoFalke

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
 -----