Web: add some technical details to the socketIO documentation

Add technical details to explain which part of the SocketIO-based broadcast
system is implemented where in the source code.

web/project-website/content/development/socketio/_index.md

@@ -16,7 +16,31 @@ SocketIO messages have an *event name* and *room name*.
 - **Manager** typically sends to all clients in a specific *room*. Which client
   has joined which room is determined by the Manager as well. By default every
   client joins the "job updates" and "chat" rooms. This is done in the
-  `OnConnection` handler defined in `registerSIOEventHandlers()`.
+  `OnConnection` handler defined in `registerSIOEventHandlers()`.  Clients can
+  send messages to the Manager to change which rooms they are in.
 - Received messages (regardless of by whom) are handled based only on their
-  *event name*. The *room name* only determines *which* client receives those
+  *event name*. The *room name* only determines *which client* receives those
   messages.
+
+## Technical Details
+
+The following files & directories are relevant to the SocketIO broadcasting
+system on the Manager/backend side:
+
+`internal/manager/webupdates`
+: package for the SocketIO broadcasting system
+
+`internal/manager/webupdates/sio_rooms.go`
+: contains the list of predefined SocketIO *rooms* and *event types*. Note that
+ there are more rooms than listed in that file; there are  dynamic room name
+ like `job-fa48930a-105c-4125-a7f7-0aa1651dcd57` that cannot be listed there as
+ constants.
+
+`internal/manager/webupdates/job_updates.go`
+: sending job-related updates.
+
+`internal/manager/webupdates/worker_updates.go`
+: sending worker-related updates.
+
+For a relatively simple example of a job update broadcast, see
+`func (f *Flamenco) SetJobPriority(...)` in `internal/manager/api_impl/jobs.go`.