DOC: Fix input type description in Interface.help()

Fixes #1000

I didn't think this would be so easy.

Note: `full_info` was added to traits in [this commit](https://github.com/enthought/traits/blob/8e298a8770c7efb52b04c8dc08e9ba397c8ad461/enthought/traits/trait_handlers.py#L297) for 3.0.