"swift -help" shows the following output for the -target and -target-variant options:
This text is incredibly vague. At minimum, it should say <triple> instead of <value> to indicate that this is a target triple; it might even make sense to include an example like "x86_64-apple-macos10.9" in the description.
Command-line options and their help are specified in "include/swift/Option/Options.td". To figure out what you need to change, take a look at how some of the options which have custom names, like -sdk, are specified.