Clarify impact of datetime coordinate unit on polyfit, polyval, integrate, differentiate

[dcherian]

What is your issue?

We will soon support non-nanosecond time units. There are a few functions whose output could rely on the spacing of coordinate values: polyfit , polyval, integrate, differentiate (are there any more?)

1. integrate and differentiate take a datetime_unit kwarg, polyfit and polyval do not.
2. polyfit and polyval force conversion to ns:

   xarray/xarray/core/computation.py

   Line 2117 in ceb5364

   data=datetime_to_numeric(x.data, offset=offset, datetime_unit="ns"),

We probably want to relax (2) in the future. Either way it would be good to systematically think about this and at least talk about it in the docstrings.