Document and clarify kernel generation models

[rafaqz]

@jamesmaino probably good if you had a look at these

https://github.com/cesaraustralia/Dispersal.jl/blob/master/src/rules/kernel/common.jl#L108-L191

[jamesmaino]

Looks fine to me. should be equivalent when no sub-sample. Can you make a comment in docs of the paper that looks into the theory of it "On the approximation of continuous dispersal kernels in discrete-space models Joseph D. Chipperfield et al 2011"