Help: Note C_STANDARD values added in CMake 3.21

Issue: #22366

Help/prop_tgt/C_STANDARD.rst

@@ -11,7 +11,26 @@ flag such as ``-std=gnu11`` to the compile line.  For compilers that
 have no notion of a C standard level, such as Microsoft Visual C++ before
 VS 16.7, this property has no effect.
 
-Supported values are ``90``, ``99``, ``11``, ``17``, ``23``.
+Supported values are:
+
+``90``
+  C89/C90
+
+``99``
+  C99
+
+``11``
+  C11
+
+``17``
+  .. versionadded:: 3.21
+
+  C17
+
+``23``
+  .. versionadded:: 3.21
+
+  C23
 
 If the value requested does not result in a compile flag being added for
 the compiler in use, a previous standard flag will be added instead.  This