QtQuick.Window: add missing documentation for Window methods

Several more methods/slots are inherited from QWindow but are not
documented for the QML Window type.  Also some other documentation
improvements.

Task-number: QTBUG-40093
Task-number: QTBUG-42426
Change-Id: Ib753be269cbc41ee540e6556e0ef483758eefe62
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>