Document keepdims

[relativistic]

What is your issue?

I wanted to keep a dim after taking the median along that dim. I couldn't find anything about how to do this from either the median api docs , nor the aggregation docs.

After some googling though I found this feature was added in a PR through the keepdims kwarg.

It would be useful for this (plus any other hidden aggregation modes) to be documented in the Aggregation and/or apis. Discovery is one thing, and clarifying ambiguities is another. For example, what happens if I use keepdims to all the coordinates that used that dim?

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