Minor GroupBy doc fixes

[TheNeuralBit]

Post-Commit Tests Status (on master branch)

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Java
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XLang		---		---		---

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Portable	---		---	---	---	---

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[TheNeuralBit]

I checked for other transform pages that are missing from the nav and came across the following incomplete, orphan pages:
https://beam.apache.org/documentation/transforms/python/elementwise/withkeys/
https://beam.apache.org/documentation/transforms/python/other/passert/
https://beam.apache.org/documentation/transforms/python/aggregation/approximatequantiles/
https://beam.apache.org/documentation/transforms/python/aggregation/approximateunique/

Seems like we should complete them or just delete them. I think they'd all be fine to delete aside from PAssert. I don't think WithKeys is used often and the Approximate transforms indicate they're deprecated.

[robertwb]

Thanks for catching this. Probably worth filing a jira for the others.