Updates documentation of page format to provide info to set unusual page sizes directly

[rolf-thomas]

I had a bit of a hard time finding this config option at all

[maechler]

@rolf-thomas Thank you for that addition! I'm actually a little surprised that this is already working partially, as the format is treated as string throughout the extension. There are however still some issues and I created an issue to address them: https://github.com/bithost-gmbh/pdfviewhelpers/issues/245