Documentation: Integral kernel

[bela127]

Hello,

I guess the 3 integral kernels, are integral kernels over a specific function prior (e.g. RBF-Integral Kernels?).
I.e. there could be an integral kernel for every kernel that can be integrated over.

For what kernel is this the integral kernel?
If you add more documentation to the kernel and the code it would be easier to contribute more (other) integral kernels.