improve to_zarr doc about chunking

[apatlpo]

• follows automatic chunking of zarr archive #4046
• Passes isort -rc . && black . && mypy . && flake8

I'm not sure the last point is really necessary for this PR, is it?

[pep8speaks]

Hello @apatlpo! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2020-05-20 13:37:26 UTC

[shoyer]

Can we instead state what does happen if a variable is already chunked or chunks can be found in encoding?

Something like

• If chunks are found in the encoding argument or attribute corresponding to any DataArray, those chunks are used.
• Otherwise, if the DataArray is already a dask array, it is written with those chunks.
• Finally, if not other chunks are found, Zarr uses its own heuristics to choose automatic chunk sizes.

[apatlpo]

I closely followed your suggestion.
Note that I am not sure about the docstring style for a list-like note like this one.

[apatlpo]

if anybody has a clue on how to fix tests this would be welcome.
thx

[rabernat]

There are two sphinx warnings in building the docs
https://dev.azure.com/xarray/xarray/_build/results?buildId=2819&view=logs&j=7e620c85-24a8-5ffa-8b1f-642bc9b1fc36&t=68484831-0a19-5145-bfe9-6309e5f7691d&l=280

xarray/core/dataset.py:docstring of xarray.Dataset.to_zarr:44: WARNING: Definition list ends without a blank line; unexpected unindent.
xarray/core/dataset.py:docstring of xarray.Dataset.to_zarr:46: WARNING: Unexpected indentation.

These seem related to your changes. I would try to get the docs building locally without any warnings.

[keewis]

the flake8 issues are fixed in #4057, so you can merge master into your branch to fix them in this branch, too.

[apatlpo]

ok, tests are passing.

[keewis]

they do, but could you remove the saved_on_disk.nc file that was accidentally added?

Also, the items are not a list despite being formatted like one:

is that intentional?

[apatlpo]

argh ... saved_on_disk.nc has been removed.

I was unfortunately not able to create a proper list without breaking tests and have exhausted my time trying to figure it out.
If you know how to do that I am more than happy to follow your advice.

[shoyer]

I think it's OK that it's not formatted as a list. It's still pretty readable

[shoyer]

thanks @apatlpo !