Update doc.

Move to the class documentation in the cpp file,
focus on documenting the class and not necessarily
high-dpi scaling in general.

Change-Id: I4f04398904325b2f9061db8d3a82cd362276e977
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@theqtcompany.com>