Use boilerplate form for VS version, /std flag

Some passive constructions can't be avoided.

Author: colin-home

docs/cpp/constexpr-cpp.md

@@ -61,7 +61,7 @@ The following rules apply to constexpr functions:
 
 - A **`constexpr`** function can be recursive.
 
-- Before C++20 it couldn't be [virtual](../cpp/virtual-cpp.md). A constructor couldn't be defined as **`constexpr`** when the enclosing class had any virtual base classes. Visual C++ supports **`constexpr`**  virtual functions since Visual Studio 2019 16.10.
+- Before C++20, a **`constexpr`** function can't be [virtual](../cpp/virtual-cpp.md), and a constructor can't be defined as **`constexpr`** when the enclosing class has any virtual base classes. In C++20 and later, a **`constexpr`** function can be virtual. Visual Studio 2019 version 16.10 and later versions support **`constexpr`** virtual functions when you specify the [`/std:c++20`](../build/reference/std-specify-language-standard-version.md) or later compiler option.
 
 - The body can be defined as `= default` or `= delete`.