Supplementing .NET Workflows fan in/out pattern documentation

[WhitWaldo]

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Description

Supplemented the documentation at the bottom of the workflow fan-out/in example to include another helper method introduced in the 1.16 release that dramatically simplifies processing work items in parallel with an upper concurrency limit. It was previously suggested in Discord that the appropriate way to do this was with Parallel.ForEachAsync, but this yields a non-deterministic process and is not compatible with Dapr Workflows, so I added this method to provide a method signature that's similar and provides a simpler expected experience, but which is fully compatible with Dapr Workflows.

Issue reference

[marcduiker]

This is a great addition, thanks!

[msfussell]

@WhitWaldo - Nice. I would encourage an example of this API to be added here https://github.com/dapr/dotnet-sdk/tree/master/examples/Workflow

We should encourage this approach into other SDKs.

[WhitWaldo]

@msfussell it's in there: https://github.com/dapr/dotnet-sdk/tree/release-1.16/examples/Workflow/WorkflowParallelFanOut