Consolidate mathematical description of kernels between docs and docstrings

[st--]

@willtebbutt wrote on #213:

I'm concerned is that we're duplicating documentation a bit at the minute by having both docstrings and having the entire Kernels section written out manually. This creates an opportunity for the docs to decouple from the docstrings and increases the maintenance burden. I wonder whether it would make sense, therefore, to consolidate this documentation -- if we were to move the concise mathematical description of the kernels from the docs to the docstrings, and just move /duplicate bits of the API section in the Kernel Functions section we could get the best of both worlds?

Originally posted by @willtebbutt in #213 (comment)

[devmotion]

This is basically what I had in mind with the suggestion in #216 as well.

[devmotion]

Closed by #236.