describe new fn layers for UHK, remove mirror layer

[mhantsch]

Describe new fn layers for UHK, remove mirror layer. The current documentation is no longer up-to-date with latest UHK config.

[mhantsch]

I have at least updated the UHK config now to the latest UHK60 config (tested and used with firmware 11.2.0; should also work on firmware 12.x).