Correct timeout remarks for SqlClient async command methods

[roji]

After investigating npgsql/npgsql#2457, it turns out that SqlCommand.CommandTimeout works well with async methods, just not with the old-style pre-async/await Begin* methods. However, the documentation remarks still indicate that timeouts aren't supported with any async SqlClient APIs.

[roji]

/cc @cheenamalhotra @David-Engel @mairaw

[roji]

Test used to confirm the behavior:

using var conn = new SqlConnection("...");
using var cmd = new SqlCommand("WAITFOR DELAY '00:00:05'", conn) { CommandTimeout = 1 };
conn.Open();

// Sync: times out
cmd.ExecuteNonQuery();

// Async: times out
await cmd.ExecuteNonQueryAsync();

// Old-style async: does not time out
var asyncResult = cmd.BeginExecuteNonQuery();
asyncResult.AsyncWaitHandle.WaitOne();
var result = cmd.EndExecuteNonQuery(asyncResult);

[roji]

Corresponding PR for Microsoft.Data.SqlClient: dotnet/SqlClient#264

[roji]

@carlossanlop the SqlClient people have approved this, and the corresponding changes have already been merged in https://github.com/dotnet/sqlclient. Can someone sign off on this from your side as well?

[carlossanlop]

@BillWagner @mairaw can someone in your team please take a look?

[mairaw]

Sure, I'll review this one. @stevestein you should keep an eye on PRs assigned to you for review for the System.Data space.

[roji]

ping, this is still waiting validation.

[carlossanlop]

Sorry for the delay @roji. Thanks for making this change. It looks good. I'll merge it.

[roji]

No problem at all, thanks @carlossanlop!