Update docs to include note about existing issues with Async APIs

[cheenamalhotra]

Include note about below issues until they get fixed:

• Reading large data (binary, text) asynchronously is extremely slow #593
• Canceling SQL Server query with while loop hangs forever #44

[DavoudEshtehari]

Thanks @cheenamalhotra for actively continuing contribution here. I'd rather introducing a new section for SqlClient's known issues under ADO.net doc.

[David-Engel]

I'd rather introducing a new section for SqlClient's known issues under ADO.net doc.

@DavoudEshtehari
Cheena discussed this with me and I initially proposed the same thing. However, given the impact of these known issues and the fact that there is no quick fix coming, making them apparent in the API docs, where devs are more likely to see them, makes it more likely that they will be able to solve their issues quickly, when they run into them.

[cheenamalhotra]

I'd rather introducing a new section for SqlClient's known issues under ADO.net doc.

You should do that too IMO for lesser popular issues but with significant impact along-with mitigations/solutions - and pull them in API docs when they become more popular and not fixable in short term.

The more awareness and mitigations driver docs can provide will eventually reduce the amount of repeated questions from customers.