document how to model a one-or-the-other parameter

From https://groups.google.com/d/msg/common-workflow-language/t7GEjKwJHys/2BvtkhsjXxMJ

```
> I'm not seeing how to model a boolean so that both 'true' and 'false' result
> in differents strings being added to the command line.

One way to do this is to define an "enum" where the symbols are the
two alternate command line flags. Another way is to use an expression
that overrides the default handling of boolean. A third way would be
to define two types with the command line flag in inputBinding.prefix
and use a union.
```