Documentation: Describe the non-prefixed "Device:*" variables.

These are the ones that people should use in build steps and such.

Task-number: QTCREATORBUG-14614
Change-Id: I398ee57a0338c16f41dd190abe3fc9b055a50132
Reviewed-by: hjk <hjk@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>