Clarify docs

Author: coretl

docs/explanations/why-use-pythonIoc.rst

@@ -48,6 +48,11 @@ allows you to write this as:
 
         from aioca import camonitor
 
+    You also need to do the ``camonitor`` from inside the dispatcher's
+    event loop:
+
+        dispatcher.loop.call_soon_threadsafe(camonitor, PVs, update_avg)
+
 Dynamically created PVs
 -----------------------