add a paragraph on async backend errors

Overall-architecture.md

@@ -175,6 +175,17 @@ server, its IP address and port is configurable in the API.
   cases it uses the frontend REST API to mark the job as failed.
   
 
+#### Asynchronous communication between broker and API
+
+Only a fraction of the errors that can happen during evaluation can be detected 
+while there is a ZeroMQ connection between the API and broker. To notify the 
+frontend of the rest, we need an asynchronous communication channel that can be 
+used by the broker when the status of a job changes (it's finished, it failed 
+permanently, the only worker capable of processing it disconnected...).
+
+This functionality is supplied by the `broker-reports/` API endpoint group -- 
+see its documentation for more details.
+
 ### File Server - Web API communication
 
 File server has a REST API for interaction with other parts of ReCodEx. Description of communication with workers is in [File server side](#file-server-side) section. On top of that, there are other commands for interaction with the API: