Doc: update Accelbubble example

The app could not recover from receiving a NaN value from
the Accelerometer, so NaN values are ignored.

Change-Id: Ia33d800f7fe080e07dcf327c26b3772e994793bf
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>

doc/src/qtquick/creator-mobile-app-tutorial.qdoc

@@ -196,6 +196,9 @@
             \skipto onReadingChanged
             \printuntil }
 
+            We want to ensure that the position of the bubble is always within
+            the bounds of the screen. If the Accelerometer returns not a number
+            (NaN), the value is ignored and the bubble position is not updated.
         \li Add SmoothedAnimation behavior on the \a x and \a y properties of
             the bubble to make its movement look smoother.