Add documentation for compute function kwargs

[f0uriest]

A number of compute functions can take keyword arguments for things like the adiabatic index, vector basis, boozer resolution etc. We should add documentation for those, maybe in a new column in https://desc-docs.readthedocs.io/en/stable/variables.html

[f0uriest]

Add docstrings as value of kwarg in register compute fun

add docs to list of variables, point compute function docstring to that