Exception message for multi-threaded access should include link to documentation

[cgillum]

As more people are trying Durable Functions, more issues are arising due to folks not reading documentation (which is not unexpected). Currently, the exception message users see is this:

System.InvalidOperationException: Multithreaded execution was detected. This can happen if the orchestrator function previously resumed from an unsupported async callback.
  at Microsoft.Azure.WebJobs.DurableOrchestrationContext.ThrowIfInvalidAccess()
  at Microsoft.Azure.WebJobs.DurableOrchestrationContext.d__47`1.MoveNext()
  End of stack trace from previous location where exception was thrown
  at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
  at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
  at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()

It's not clear enough what the user should do in this case. We should improve this in a couple ways:

• Make the exception less vague about what could be causing the problem. Specific suggestions would be more helpful, like saying This can happen if the orchestrator function code awaits on a task that was not created by a DurableOrchestrationContext method.
• Have a link this documentation.

These exception message changes should improve the debugging experience and help reduce support costs.

[cgillum]

This fix will go out with the next release.