Fix type signatures on numba guvectorize decorator

The type info currently there doesn't make much sense: https://github.com/arviz-devs/xarray-einstats/blob/main/src/xarray_einstats/numba.py#L13-L22

Open src/xarray_einstats/numba.py and inspect lines 13–22, then check how the guvectorize decorator's type information is defined and used. Update the signatures so they accurately describe the decorator, and verify the resulting annotations against the surrounding module behavior.

Written by the indexing model from the issue text.