For VS generator names that do not specify the platform name, read
CMAKE_GENERATOR_PLATFORM to get it.
Extend the RunCMake.GeneratorPlatform test with a case covering
use of the x64 platform when the test generator is a Visual Studio
generator whose name does not specify a platform.
Run "cmake --help-full cmake.1.rst" by hand on Windows and OS X.
Copy the generator/*.rst documents for generators unique to those
platforms into Help/generator.