Update docs to reflect open_mfdataset default chunk behaviour

[jemmajeffree]

Looking at #5704 and #9038 (and various other issues that reference these), it seems like there's no easy solution and the current open_mfdataset chunk default is going to stay for a while. i.e. chunks=None as a kwarg to open_mfdataset is overwritten with chunks={} before calling open_dataset, so the default chunks are disk chunks not single-file chunks.

xarray/xarray/backends/api.py

Line 1677 in 5ce69b2

open_kwargs = dict(engine=engine, chunks=chunks or {}, **kwargs)

In which case, we should probably update the docs to reflect this current behaviour.

[welcome]

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