[doc] Document when groupBy can deadlock because of backpressure

[NamrataGuptaRoy]

Adds documentation for the 4 groupBy overloads about the scenario where backpressure can cause a deadlock with groupBy and suggesting how it could be avoided.
Adds example of deadlock with groupBy and concatMap in the reference documentation.

See #3443

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[chemicL]

Thank you. I have some formatting and style remarks but I'll add them as a separate commit.

[chemicL]

@NamrataGuptaRoy ef152df contains my alterations - mostly formatting and styling. One thing I wanted to clarify was the notion of consumption rate. I believe that term would have been misleading, as the rate didn't have any significance in the scenarios provided - it was the consumption pattern (concurrency too low) that prevented progress. The rate could have been enormous and nothing would have changed :)