Clarify std::async policy behavior

[Mq-b]

Update async policy description to align with C++ standard and clarify Microsoft implementation

• Modified the description of launch::async policy to reflect that the C++ standard states the function behaves as if it invokes the callable object in a new thread.
• Clarified that the Microsoft implementation uses the Windows ThreadPool, which may provide recycled threads, resulting in behavior equivalent to launch::async|launch::deferred.
• Added a note about the implications for thread-local variables, recommending against their use with async.

This update ensures the documentation accurately reflects both the C++ standard and the specifics of the Microsoft implementation.

References:

• https://eel.is/c++draft/futures.async
• https://en.cppreference.com/w/cpp/thread/launch

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[Mq-b]

I think the main thing is that C++ doesn't specify:

The C++ standard states that if policy is launch::async, the function creates a new thread.

[Mq-b]

@frederick-vs-ja

[Court72]

@TylerMSFT

Can you review the proposed changes?

Important: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team

[TylerMSFT]

Thank you

[TylerMSFT]

#sign-off