Docs: Clarify arguments to mdmultispec, mdslepian and gpss

[Andy Nowacki]

These functions are well documented, but it is confusing when reading the list of arguments as to which are positional arguments and which are keyword arguments. I suggest splitting them into two different sections.

[Charlotte Haley]

Docstrings updated here.

[Andy Nowacki]

This addresses my suggestion and can be closed.