Add automatic docstrings to functions in API module

[giordano]

Now that we have full low-level API, I think it'd be nice to have it in the documentation. But this would require attaching a docstring (with link to MPICH docs?) to the automatically generated methods, which at the moment don't have a type signature. No idea how feasible is that with Clang.jl.

CC: @t-bltg

[t-bltg]

Fair enough, I'll see what I can do (we systematically think about the docs later 😆).