Fix bugs and improve wording

source/intro.rst

@@ -355,23 +355,29 @@ by the user, for example:
         self.counter += 1
         self.log.INFO("*** periodicAction : counter = " + str(self.counter))
 
-The worker can then be stopped like this:
+The worker can then be stopped by overriding ``preDestruction`` method,
+like this:
 
 .. code-block:: python
 
-    if self.worker is not None:
-        if self.worker.is_running():
-            self.worker.stop()
-        self.worker.join()
-        self.worker = None
+    def preDestruction(self):
+    	if self.worker is not None:
+            if self.worker.is_running():
+                self.worker.stop()
+            self.worker.join()
+            self.worker = None
+
+This method will be automatically called during device shutdown process.
 
 
-Usage of "Threading" module
+Usage of "threading" module
 ===========================
 
-Sometimes it is simplier to use multithreading directly for executing
-periodic tasks in python karabo device ``Klass``.  First import
-``threading`` module.
+Sometimes if you would decide to use multithreading directly for executing
+periodic tasks in python karabo device like ``Klass`` you may follow these
+steps.
+
+First import ``threading`` module.
 
 .. code-block:: python
 
@@ -388,6 +394,7 @@ Then define class function that will do periodic work as a thread
 .. code-block:: python
 
     def polling(self):
+        self.running = True
         while self.running:
             # do some useful work like
             # polling the hardware ...
@@ -401,7 +408,6 @@ Then define method that starts ``polling`` thread ...
     def start_polling(self):
         # use state here that fits your needs
         self.updateState(State.MONITORING)
-        self.running = True
         if self.pollingThread is None:
             self.pollingThread  threading.Thread(target=self.polling)
             self.log.INFO("Start polling thread")
@@ -411,10 +417,9 @@ The ``start_polling`` may be placed into ``initialize`` function, into
 slot of ``start`` button or another function that should activate
 the periodic task.
 
-Attention!  If this device is killed the ``slotKillDevice`` is called,
-which calls ``preDestruction`` method in turn.   so it is important to
-re-implement ``preDestruction`` method in ``Klass`` class to stop
-running our polling loop above
+If this device is killed the ``slotKillDevice`` is called,
+which calls ``preDestruction`` method.   So it is important to
+override this method to stop running our polling loop above
 
 .. code-block:: python