Docs for undocumented functions

[agladysh]

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We're iteratively documenting our Katamari:

https://github.com/lua-nucleo/lua-nucleo/

As you can see, there is a lot of empty pages in the documentation.

We'd like to see at least list of names of undocumented functions that a module exports. Is it possible? (Probably shouldn't be on by default, should be an option in LDoc config.)