Help: Document valid 14 value for CXX_STANDARD. (#15339)

Support was added in commit v3.1.0-rc1~475^2 (Features: Add support
for C++14 features., 2014-05-06), but the documentation for this
property was not amended.

Help/prop_tgt/CXX_STANDARD.rst

@@ -7,7 +7,7 @@ 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=gnu++11`` to the compile line.
 
-Supported values are ``98`` and ``11``.
+Supported values are ``98``, ``11`` and ``14``.
 
 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