Clarify redis monitoring docs. Closes #1272

docs/userguide/monitoring.rst

@@ -385,13 +385,19 @@ The default queue is named `celery`. To get all available queues, invoke:
 
 .. note::
 
-  If a list has no elements in Redis, it doesn't exist. Hence it won't show up
-  in the `keys` command output. `llen` for that list returns 0 in that case.
-
-  On the other hand, if you're also using Redis for other purposes, the output
-  of the `keys` command will include unrelated values stored in the database.
-  The recommended way around this is to use a dedicated `DATABASE_NUMBER` for
-  Celery.
+    Queue keys only exists when there are tasks in them, so if a key
+    does not exist it simply means there are no messages in that queue.
+    This is because in Redis a list with no elements in it is automatically
+    removed, and hence it won't show up in the `keys` command output,
+    and `llen` for that list returns 0.
+
+    Also, if you're using Redis for other purposes, the
+    output of the `keys` command will include unrelated values stored in
+    the database.  The recommended way around this is to use a
+    dedicated `DATABASE_NUMBER` for Celery, you can also use
+    database numbers to separate Celery applications from each other (virtual
+    hosts), but this will not affect the monitoring events used by e.g. Flower
+    as Redis pub/sub commands are global rather than database based.
 
 .. _monitoring-munin: