Help: Clarify what MSVC compilers ignore <LANG>_STANDARD_REQUIRED

Fixes #20953

Help/prop_tgt/CUDA_STANDARD_REQUIRED.rst

@@ -10,7 +10,7 @@ If this property is set to ``ON``, then the value of the
 property is ``OFF`` or unset, the :prop_tgt:`CUDA_STANDARD` target property is
 treated as optional and may "decay" to a previous standard if the requested is
 not available.  For compilers that have no notion of a standard level, such as
-MSVC, this has no effect.
+MSVC 1800 (Visual Studio 2013) and lower, this has no effect.
 
 See the :manual:`cmake-compile-features(7)` manual for information on
 compile features and a list of supported compilers.

Help/prop_tgt/CXX_STANDARD_REQUIRED.rst

@@ -10,7 +10,7 @@ If this property is set to ``ON``, then the value of the
 property is ``OFF`` or unset, the :prop_tgt:`CXX_STANDARD` target property is
 treated as optional and may "decay" to a previous standard if the requested is
 not available.  For compilers that have no notion of a standard level, such as
-MSVC, this has no effect.
+MSVC 1800 (Visual Studio 2013) and lower, this has no effect.
 
 See the :manual:`cmake-compile-features(7)` manual for information on
 compile features and a list of supported compilers.

Help/prop_tgt/C_STANDARD_REQUIRED.rst

@@ -10,7 +10,7 @@ If this property is set to ``ON``, then the value of the
 property is ``OFF`` or unset, the :prop_tgt:`C_STANDARD` target property is
 treated as optional and may "decay" to a previous standard if the requested is
 not available.  For compilers that have no notion of a standard level, such as
-MSVC, this has no effect.
+MSVC 1800 (Visual Studio 2013) and lower, this has no effect.
 
 See the :manual:`cmake-compile-features(7)` manual for information on
 compile features and a list of supported compilers.