Help: --build-target can be repeated in ctest --build-and-test mode

I discovered this by accident after wanting to build-and-test multiple targets for header set verification. Looking at the implementation of cmCTestBuildAndTestHandler::RunCMakeAndTest(), it explicitly loops over the targets accumulated in cmCTestBuildAndTestHandler::ProcessCommandLineArguments(), so it appears the lack of it being mentioned as supported in the documentation was just an oversight.