moving apidiff out of preview (#3595)

release-notes/3.0/api-diff/.Net/3.0.0_System.Threading.md

@@ -0,0 +1,45 @@
+# System.Threading
+
+``` diff
+ namespace System.Threading {
+-    public struct AsyncLocalValueChangedArgs<T>
++    public readonly struct AsyncLocalValueChangedArgs<T>
+     public readonly struct CancellationToken {
++        public CancellationTokenRegistration UnsafeRegister(Action<object> callback, object state);
+     }
+-    public readonly struct CancellationTokenRegistration : IDisposable, IEquatable<CancellationTokenRegistration> {
++    public readonly struct CancellationTokenRegistration : IAsyncDisposable, IDisposable, IEquatable<CancellationTokenRegistration> {
++        public ValueTask DisposeAsync();
++        public bool Unregister();
+     }
++    public interface IThreadPoolWorkItem {
++        void Execute();
++    }
+     public static class Monitor {
++        public static long LockContentionCount { get; }
+     }
+     public sealed class PreAllocatedOverlapped : IDisposable {
++        ~PreAllocatedOverlapped();
+     }
+     public sealed class ReaderWriterLock : CriticalFinalizerObject {
+-        ~ReaderWriterLock();
+
+     }
+     public struct SpinWait {
++        public void SpinOnce(int sleep1Threshold);
+     }
+     public static class ThreadPool {
++        public static long CompletedWorkItemCount { get; }
++        public static long PendingWorkItemCount { get; }
++        public static int ThreadCount { get; }
++        public static bool UnsafeQueueUserWorkItem(IThreadPoolWorkItem callBack, bool preferLocal);
++        public static bool UnsafeQueueUserWorkItem<TState>(Action<TState> callBack, TState state, bool preferLocal);
+     }
+-    public sealed class Timer : MarshalByRefObject, IDisposable {
++    public sealed class Timer : MarshalByRefObject, IAsyncDisposable, IDisposable {
++        public static long ActiveCount { get; }
++        public ValueTask DisposeAsync();
+     }
+ }
+```
+