Update std-specify-language-standard-version.md

Add info about what to set __cplusplus to

Author: TylerMSFT

docs/build/reference/std-specify-language-standard-version.md

@@ -28,7 +28,12 @@ The Microsoft C++ compiler in Visual Studio 2017 and later versions doesn't supp
 
 The **`/std`** option in effect during a C++ compilation can be detected by use of the [`_MSVC_LANG`](../../preprocessor/predefined-macros.md) preprocessor macro. For more information, see [Preprocessor Macros](../../preprocessor/predefined-macros.md).
 
-The [`/Zc:__cplusplus`](zc-cplusplus.md) option must additionally be used for the `__cplusplus` macro to be correctly defined for the appropriate C++ standard. 
+> [!IMPORTANT]
+> Because some existing code depends on the value of the macro `__cplusplus` being 199711L, the MSVC compiler doesn't change the value of this macro unless you explicitly opt in by setting [`/Zc:__cplusplus`](zc-cplusplus.md). Here are the values for `__cplusplus` depending on the language version you are compiling with:
+> | Language version | __cplusplus value |
+> | `/std:c++14` | 201402 |
+> | `/std:c++17` | 201703 |
+> | `/std:c++20` | 202002 |
 
 **`/std:c++14`**\
 The **`/std:c++14`** option enables C++14 standard-specific features implemented by the MSVC compiler. This option is the default for code compiled as C++. It's available starting in Visual Studio 2015 Update 3.