Libmongocrypt readme contains instructions for building documentation in https://github.com/mongodb/libmongocrypt/#documentation:

> The documentation can be rendered into HTML with doxygen. Run doxygen ./doc/Doxygen, then open ./doc/html/index.html.

I've done that and received what appears to be empty documentation. Perhaps this is because the header file with the docstrings now has a .h.in extension and is ignored/not parsed by doxygen.