Help: Document preference of `MSVC_VERSION` over `MSVC##`

Issue: #16735

Help/variable/MSVC.rst

@@ -4,3 +4,5 @@ MSVC
 ``True`` when using Microsoft Visual C++.
 
 Set to ``true`` when the compiler is some version of Microsoft Visual C++.
+
+See also the :variable:`MSVC_VERSION` variable.

Help/variable/MSVC10.rst

 MSVC10
 ------
 
+Discouraged.  Use the :variable:`MSVC_VERSION` variable instead.
+
 ``True`` when using the Microsoft Visual Studio ``v100`` toolset
 (``cl`` version 16) or another compiler that simulates it.

Help/variable/MSVC11.rst

 MSVC11
 ------
 
+Discouraged.  Use the :variable:`MSVC_VERSION` variable instead.
+
 ``True`` when using the Microsoft Visual Studio ``v110`` toolset
 (``cl`` version 17) or another compiler that simulates it.

Help/variable/MSVC12.rst

 MSVC12
 ------
 
+Discouraged.  Use the :variable:`MSVC_VERSION` variable instead.
+
 ``True`` when using the Microsoft Visual Studio ``v120`` toolset
 (``cl`` version 18) or another compiler that simulates it.

Help/variable/MSVC14.rst

 MSVC14
 ------
 
+Discouraged.  Use the :variable:`MSVC_VERSION` variable instead.
+
 ``True`` when using the Microsoft Visual Studio ``v140`` or ``v141``
 toolset (``cl`` version 19) or another compiler that simulates it.

Help/variable/MSVC60.rst

 MSVC60
 ------
 
+Discouraged.  Use the :variable:`MSVC_VERSION` variable instead.
+
 ``True`` when using Microsoft Visual C++ 6.0.
 
 Set to ``true`` when the compiler is version 6.0 of Microsoft Visual C++.

Help/variable/MSVC70.rst

 MSVC70
 ------
 
+Discouraged.  Use the :variable:`MSVC_VERSION` variable instead.
+
 ``True`` when using Microsoft Visual C++ 7.0.
 
 Set to ``true`` when the compiler is version 7.0 of Microsoft Visual C++.

Help/variable/MSVC71.rst

 MSVC71
 ------
 
+Discouraged.  Use the :variable:`MSVC_VERSION` variable instead.
+
 ``True`` when using Microsoft Visual C++ 7.1.
 
 Set to ``true`` when the compiler is version 7.1 of Microsoft Visual C++.

Help/variable/MSVC80.rst

 MSVC80
 ------
 
+Discouraged.  Use the :variable:`MSVC_VERSION` variable instead.
+
 ``True`` when using the Microsoft Visual Studio ``v80`` toolset
 (``cl`` version 14) or another compiler that simulates it.

Help/variable/MSVC90.rst

 MSVC90
 ------
 
+Discouraged.  Use the :variable:`MSVC_VERSION` variable instead.
+
 ``True`` when using the Microsoft Visual Studio ``v90`` toolset
 (``cl`` version 15) or another compiler that simulates it.