I there comprehensive, all in one place, ^^^subject^^^?
While there is an architectural overview for ucode APIs, for the most part you will have to check the source and its inline comments. Luci made a continuous in-place transition from server-side lua scripts to offloading a lot of the work to client-side ECMAscript handling with a ucode backend in rather short time, so things certainly have changed massively within ~2 years (abandoning prior attempts), so the existing source is your best bet.
I usually use https://openwrt.github.io/luci/api/index.html to get overview information, then dive into the code.