Clean up async API docs for System.Data

[roji]

The current API docs for System.Data async methods simply point to the sync method, and provide information on the default implementation (throw on cancellation, invoke the sync method...). We should instead have the full information on the method directly available on the async method, and provide any default implementation info in the remarks.

Example method: OpenAsync.

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[bgrainger]

@danmosemsft Can I be subscribed to area-System.Data issues?

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