Help/cmake-cxxmodules: clarify stdlib support for Clang by name

Rather than by flag which might imply that an explicit flag is
necessary.

Help/manual/cmake-cxxmodules.7.rst

@@ -80,7 +80,7 @@ modules includes:
 Support for ``import std`` is limited to the following toolchain and standard
 library combinations:
 
-* Clang 18.1.2 and newer with ``-stdlib=libc++`` or ``-stdlib=libstdc++``
+* Clang 18.1.2 and newer with standard library ``libc++`` or ``libstdc++``
 * MSVC toolset 14.36 and newer (provided with Visual Studio 17.6 and newer)
 * GCC 15 and newer