Csharp language reference attributes: add missing AsyncMethodBuilder SetStateMachine member (#45929)

This method is mentioned in the Roslyn documentation:

https://github.com/dotnet/roslyn/blob/main/docs/features/task-types.md

If it is not included when implementing the AsyncMethodBuilder  pattern, the C# compiler fails to compile with a CS0656 error.

I also added a `public` to the `Start` method to match the other methods described in this section.

Author: AustinWise

docs/csharp/language-reference/attributes/general.md

@@ -147,10 +147,16 @@ The constructor to the `AsyncMethodBuilder` attribute specifies the type of the
 * A `Start` method with the following API signature:
 
   ```csharp
-  void Start<TStateMachine>(ref TStateMachine stateMachine)
+  public void Start<TStateMachine>(ref TStateMachine stateMachine)
             where TStateMachine : System.Runtime.CompilerServices.IAsyncStateMachine
   ```
 
+* A `SetStateMachine` method with the following API signature:
+
+  ```csharp
+  public void SetStateMachine(System.Runtime.CompilerServices.IAsyncStateMachine stateMachine)
+  ```
+
 * An `AwaitOnCompleted` method with the following signature:
 
   ```csharp