More `.. versionadded` could be added later when the features,
variables and properties relevant to each generator are properly
documented.
Issue: #19715
Extend the `-T <toolset>` option to support a `buildsystem=` field with
the Xcode generator. Add a `CMAKE_XCODE_BUILD_SYSTEM` variable to
inform project code about the selected build system variant.
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.