improve docs on zarr + cloud storage

[rabernat]

In the Pangeo gitter chat, @birdsarah helped identify some shortcomings in the documentation about zarr cloud storage (https://github.com/pydata/xarray/blob/master/doc/io.rst#cloud-storage-buckets). We don't mention s3fs or how to use authentication. A more detailed set of examples would probably help people struggling to make the pieces fit together.

[stale]

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