Using `&= help` with `&= argPos` throws an error

[aviaviavi]

I can use argPos just fine, but if I add a help param, I get

CallStack (from HasCallStack):
  error, called at ./System/Console/CmdArgs/Implicit/Local.hs:106:11 in cmdargs-0.10.20-F07nW8QLYyL7Sb2sj6I5zi:System.Console.CmdArgs.Implicit.Local

Is there another way I'm supposed to be documenting positional arguments? If so, a better error message and docs to show this would be helpful, ideally it shouldn't type check.

I'm using stack with ghc 8.4.3, if it matters

[aviaviavi]

If this is a small contained thing to fix that no one else is or has been working on, I'd be happy to take a swing at it, let me know!