Document std::mutex constructor nonconformance

docs/standard-library/mutex-class-stl.md

@@ -1,7 +1,7 @@
 ---
 description: "Learn more about: mutex Class (C++ Standard Library)"
 title: "mutex Class (C++ Standard Library)| Microsoft Docs"
-ms.date: "11/04/2016"
+ms.date: "20/10/2021"
 f1_keywords: ["mutex/std::mutex", "mutex/std::mutex::mutex", "mutex/std::mutex::lock", "mutex/std::mutex::native_handle", "mutex/std::mutex::try_lock", "mutex/std::mutex::unlock"]
 ms.assetid: 7999d055-f74f-4303-810f-8d3c9cde2f69
 helpviewer_keywords: ["std::mutex [C++]", "std::mutex [C++], mutex", "std::mutex [C++], lock", "std::mutex [C++], native_handle", "std::mutex [C++], try_lock", "std::mutex [C++], unlock"]
@@ -57,9 +57,11 @@ If the calling thread already owns the `mutex`, the behavior is undefined.
 Constructs a `mutex` object that is not locked.
 
 ```cpp
-constexpr mutex() noexcept;
+mutex() noexcept;
 ```
 
+**Note:** according to C++ Standard, this constructor should be declared as constexpr, currently it isn't due to compatibility reason.
+
 ## <a name="dtormutex_destructor"></a> mutex::~mutex Destructor
 
 Releases any resources that are used by the `mutex` object.