explanation of cases when broker needs to send messages to the monitor

Implementation.md

@@ -1187,9 +1187,9 @@ discarded.
 #### Commands from Broker to Monitor:
 
 - **progress** -- notification about progress with job evaluation. This
-  communication is usually redirected as is from worker, more info can be found
-  in "Broker - Worker Communication" chapter above.
-
+  communication is usually redirected as is from worker (the exception being
+  cases when the worker might have crashed without sending a failure notice),
+  more info can be found in "Broker - Worker Communication" chapter above.
 
 ### Broker - REST API Communication