Document xcode_workspace option for Objective-C

Just a small doc edit that might save others a few minutes. The Objective-C documentation indicates that you need to provide, "the name of your scheme and either project or workspace," with the example showing the use of `xcode_project` option. We weren't sure whether we should pass an Xcode workspace to the `xcode_project` option, if there was an `xcode_workspace` option to use, or if we needed to override the `script` to specify the `-workspace` flag to xctool.