Need to improve API documentation

As reported by @McNally in https://kea.isc.org/ticket/5422:

On ISC Support ticket #12056, one of our Kea customers has requested:

..our developers commented to me that they found it difficult to find
the appropriate syntax for various API calls.

There's more on the support ticket, but it sounds as though a general API document would be helpful to them, at least containing calling specifications and pointers towards whatever other documentation currently exists.

They recognize that this is not a short-term deliverable but would find improvement in this area helpful.