Add docs on using a query in XMLRole with the less obvious boolean()

Added to a documentation snippet since a direct boolean map is useful
in QML for toggles and using "@wanted/boolean()" as the @type example
does, would be wrong.

Change-Id: If412ec0f4891db39fe1a219f9cb89e1f7103fd38
Signed-off-by: David Greaves <david@dgreaves.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>