exercise util: document input type for process_X_case

[coriolinus]

When generating process_X_case functions, we should generate documentation for the implementer about the expected input items.

This feature is not very useful useful right now, because it will tend to generate comments like

// expected input format: (input, )

As such, this issue should probably wait until https://github.com/exercism/rust/issues/681 is resolved.

However, once we implement input struct unpacking, it should say:

// expected input format: (phrase, )

And once we implement unpacking single-item tuples, it should say:

// expected input format: phrase