L2SS-245: add paragraph about manual polling

devices/test/devices/automatic_polling_performance_test/Tango_Controls-Automatic_polling_performance_test.md

@@ -77,7 +77,11 @@ The two solutions above are mutually exclusive. Other solutions that alleviate t
 - Lower the polling rate so that the polling thread has more time to perform calling the attribute `read` functions. This means that an attribute's read function is allowed to take longer to perform its tasks. Note that this does not solve the original problem that the `read` function is just too slow when it is called.
 - Move the updating of attribute values to their own process. Then the read function can return the current value immediately because the value gets independently updated.
 
-Executive summary: even Tango Controls cannot perform magic. High volume attributes cannot be polled and infinite velocity. One has to distribute the polling load either over more DS, more threads within a DS or more processes within a DS.
+There is an entirely different way to lower the pressure on the automatic polling: manual polling, i.e. performing everything that the polling thread does for a selected set of attributes. Tango Controls allows to manually send events. This opens the possibility to perform the reading of values over communication links, checking for value changes and sending out of archive or on-change events from separate threads or processes when it is convenient.
+
+# Executive summary
+
+Even Tango Controls cannot perform magic. High volume attributes cannot be polled and infinite velocity. One has to distribute the polling load either over more DS, more threads within a DS or more processes within a DS.
 
 # Data