[Documentation] Missing documentation on the enum parsing feature

[alb-i986]

I'm a google-http-client-jackson2 user, and I struggled a bit looking for a way to setup an enum type as a @Key.

This is the exception I was getting: IllegalArgumentException: given enum name New not part of enumeration

Eventually I was able to find that out myself by looking into the class FieldInfo, and I learned of the existence of the annotations @Value and @NullValue.

I'm opening this issue to please ask you to update the Developer's Guide (the JSON page in particular) with a new paragraph talking about those two annotations.

Thanks.

PS: I think this relates to #127