Help: Document CMAKE_VS_PLATFORM_NAME variable

Help/manual/cmake-variables.7.rst

@@ -73,6 +73,7 @@ Variables that Provide Information
    /variable/CMAKE_VS_INTEL_Fortran_PROJECT_VERSION
    /variable/CMAKE_VS_MSBUILD_COMMAND
    /variable/CMAKE_VS_MSDEV_COMMAND
+   /variable/CMAKE_VS_PLATFORM_NAME
    /variable/CMAKE_VS_PLATFORM_TOOLSET
    /variable/CMAKE_XCODE_PLATFORM_TOOLSET
    /variable/PROJECT_BINARY_DIR

Help/variable/CMAKE_VS_PLATFORM_NAME.rst

@@ -0,0 +1,7 @@
+CMAKE_VS_PLATFORM_NAME
+----------------------
+
+Visual Studio target platform name.
+
+VS 8 and above allow project files to specify a target platform.
+CMake provides the name of the chosen platform in this variable.