Doc: Updated the Calendar example documentation

Added more information about what the example does and how
Also updated the screenshot of the example.

Task-number: QTBUG-37203
Change-Id: I20f9e5ed864b2a1f030736ad45c495118c19bf55
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>