Doc: Use the screenshot in the Declarative Radio example documentation

Also, crop the screenshot image to not show any window decoration,
and fix the example \brief.

Change-Id: I86310bdc5a4f124180eac1a1d27064286ba34f59
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>

examples/multimedia/declarative-radio/doc/src/declarative-radio.qdoc

@@ -28,11 +28,13 @@
 /*!
     \example declarative-radio
     \title Declarative Radio Example
-    \brief Demonstrates the radio functionality
+    \brief Demonstrates using the Radio QML type from Qt Multimedia.
     \ingroup multimedia_examples
 
     This examples uses the \l{Qt Multimedia} \l Radio QML type to list the
     available channels on the FM frequency.
 
+    \image declarative-radio-example.png
+
     \include examples-run.qdocinc
 */