Help: Clarify that no version of MSVC today support C standard levels

The current description sounds as if MSVC later than 2015 update 3
support C standard level, which is not true. This patch clarifies MSVC
support.

Help/prop_tgt/C_STANDARD.rst

@@ -6,8 +6,8 @@ The C standard whose features are requested to build this target.
 This property specifies the C standard whose features are requested
 to build this target.  For some compilers, this results in adding a
 flag such as ``-std=gnu11`` to the compile line.  For compilers that
-have no notion of a standard level, such as Microsoft Visual C++ before
-2015 Update 3, this has no effect.
+have no notion of a C standard level, such as all versions of
+MSVC known as of this version of CMake, this has no effect.
 
 Supported values are ``90``, ``99`` and ``11``.