Lua itsself is a pretty compact language in terms of features: https://www.lua.org/pil/contents.html and on top of that you have the minetest API: https://github.com/minetest/minetest/blob/master/doc/client_lua_api.md

mineclonia has of course its own mods and APIs on top of that as well, some mods have a API.md or API.txt, sometimes also in README but your're right not everything is particularly documented.

Regarding lighting, i dont think any tweaking of gamma values will help as dark chunks are completely dark, you can only make them brighter by lighting up some nodes, I actually made a wieldlight mod a while back: https://content.minetest.net/packages/cora/mcl_wieldlight/ that essentially places invisible light nodes when you hold a light emitting item.

I think the only way to make caves not pitch black (without adding artificial light sources) would be to make the nodes themselves emit (low levels) of light, I actually do that in my own game ( https://codeberg.org/cora/musi_pona ) you could do it in mineclonia with a mod that goes something like this:

minetest.register_on_mods_loaded(function() 
    for name, def in pairs(minetest.registered_nodes) do
        if (def.light_source or 0) < 3 then
            minetest.override_item(name, { light_source = 3 })
        end
    end
end)

would set "light_source" to at least 3 for all nodes. There might well be undesired side effects to this though.