Also document the expected range and default values, as well as some
details about why you may want to use a probeHorizon.

This also makes sure that probeHorizon is taken into consideration when
rendering PrincipledMaterial.

Change-Id: Ie06a492d43642f6dee0634a7836f40a4d0334c31
Reviewed-by: Christian Strømme <christian.stromme@qt.io>