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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[bitsandfoxes]

@jamescrosswell do you have any thought on this?

[mattico]

Taking another crack at it:

When Global Mode is **enabled**, a single scope stack is shared by the whole application. This is the recommended mode for UI applications.

When Global Mode is **disabled**, the scope data is tied to the currently executing context. This is the recommended mode for server applications.

Since version 5.0.0 the scope's `Transaction` is always tied to the currently executing context, regardless of the Global Mode configuration.

<Alert level="info">
What does it mean for data to be "tied to the currently executing context?" In general, each async `Task`, `Thread`, or web request
has its own ExecutionContext. Data which is written to this context is visible to child tasks or threads, but not to parent tasks
or threads.

For more information see [the .NET ExecutionContext documentation](https://learn.microsoft.com/en-us/dotnet/api/system.threading.executioncontext).
</Alert>

[jamescrosswell]

Thanks for the PR @mattico! Much appreciated ❤️

I've suggested one small tweak to the wording on the last paragraph but otherwise all good!

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[bitsandfoxes]

Thanks a lot! And thanks for your (collective) patience!