Improve cancel1.cs how-to example

[Kissaki]

I attempted to fix #27830, but as per my comment there, I don't think the original raised concern is correct. I did make some improvements though, which are present in this changeset I am submitting.

This snippet is being used in "How to: Cancel a Task and Its Children" https://learn.microsoft.com/en-us/dotnet/standard/parallel-programming/how-to-cancel-a-task-and-its-children

The mixed four-fold goals of the how-to make it a complex mixed bag of an example.

• For distinction, label tasks as cancellable, parent, and child - in output and variable names
• Use output prefixes to indicate in which context the message is logged
• Reduce wait spins (it took infeasibly long on my PC)
• Reduce parent spawn-loop-spin-waits so spawning happens faster than child task execution
• Add "ran to completion" messages so execution can be followed and user cancel can be timed and experimented with at runtime
• Use CancelAsync instead of Cancel on tokenSource

[Kissaki]

CI

Error: D:\a\docs\docs\samples\snippets\csharp\VS_Snippets_Misc\tpl_cancellation\cs\cancel1.cs(64,31): error CS1061: 'CancellationTokenSource' does not contain a definition for 'CancelAsync' and no accessible extension method 'CancelAsync' accepting a first argument of type 'CancellationTokenSource' could be found (are you missing a using directive or an assembly reference?) [D:\a\docs\docs\samples\snippets\csharp\VS_Snippets_Misc\tpl_cancellation\cs\Project.csproj::TargetFramework=net6.0]

So I guess it should remain Cancel.

[BillWagner]

Thanks @Kissaki

These changes all look good. I'll now.