dotnet: clarify global mode transaction state

[mattico]

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Following getsentry/sentry-dotnet#3596, the scope Transaction is always stored in an AsyncLocal, regardless of the GlobalMode configuration. Updating the docs to state this will help others avoid needing to read the SDK source in order to figure out why their transactions become unset on the scope during task switches.

I'm not completely sure about the wording. The nice thing about ExecutionContext and the GlobalMode defaults is that in the vast majority of cases they work intuitively, and ExecutionContext is an implementation detail that users can ignore. But in those special cases I think it's helpful to explain what's actually happening to help the user understand it.

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