config refactor: every function table entry should be documented and unit tested

We've added a lot of function tables, but some of their entries are all NULL, everywhere in the code.

So even if it looks like we have 100% coverage, we're not testing these code paths.

Ideally, we should have non-trivial functions, which do the thing that the function table entry is mean to do.

Edited to add:

Also, function table types should be documented with the same level of detail as regular functions. Each argument should have a type, name?, and description.