Add standards proposal detail.

Author: Colin Robertson

docs/error-messages/compiler-warnings/c5208.md

@@ -20,7 +20,7 @@ Unnamed classes within a `typedef` declaration can't have any members other than
 
 The same restrictions are applied recursively to each nested class. The restriction is meant to ensure the simplicity of structs that have `typedef` names for linkage purposes. They must be simple enough that no linkage calculations are necessary before the compiler gets to the `typedef` name for linkage.
 
-This warning is new in Visual Studio 2019 version 16.6. Since this change was adopted by the C++ standards committee as a defect report, it affects all standards modes of the compiler. In default (**`/std:c++14`**) and **`/std:c++17`** modes, the compiler emits warning C5208 for non-conforming code. If **`/permissive-`** is specified, the compiler emits warning C5208 as an error under **`/std:c++14`** and emits error C7626 under **`/std:c++17`**. The compiler emits error C7626 for non-conforming code when **`/std:c++latest`** is specified.
+This warning is new in Visual Studio 2019 version 16.6. It's based on [P1766R1](https://wg21.link/P1766R1), adopted by the C++ standards committee as a defect report, and it affects all standards modes of the compiler. In default (**`/std:c++14`**) and **`/std:c++17`** modes, the compiler emits warning C5208 for non-conforming code. If **`/permissive-`** is specified, the compiler emits warning C5208 as an error under **`/std:c++14`** and emits error C7626 under **`/std:c++17`**. The compiler emits error C7626 for non-conforming code when **`/std:c++latest`** is specified.
 
 ### To turn off the warning without code changes