Mechanisms for more useful usage text

[vitling]

I see two main areas to improve

1. Allow FieldParsers to be self-describing, so they can give hints as to the format in which they expect input.
2. Field-level annotations to add a wordier description.

I'd say (1) is highest priority, as it creates benefit for the general case; whereas (2) increases the complexity of the API and only helps in specific cases. Both should probably be possible though.

[vitling]

Actually did (2) first, via the Hint mechanism