Document drop_variables in open_mfdataset

[jerabaul29]

Document drop_variables in open_mfdataset. Makes it even clearer that for more information about possible additional options to open_mfdataset, one can consult open_dataset. Solves the comment at #8074 (comment) .

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[jerabaul29]

(I do not really understand while these tests fail, but I suppose this is not related to the minor documentation update I just pushed?)

[TomNicholas]

Thanks for opening a PR @jerabaul29!

So what I was suggesting was actually to add a new example to the examples section of the open_mfdataset docstring, the section that starts with

Examples
--------

i.e. add a small code snippet demonstrating passing on the of the open_dataset arguments down through open_mfdataset.

[jerabaul29]

Thanks for your patience and kind explanation, sorry for misunderstanding at first. Does this look better? :)

[TomNicholas]

This looks great, thanks @jerabaul29!