[Documentation] Missing documentation on the enum parsing feature

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](https://github.com/googleapis/google-http-java-client/blob/main/google-http-client/src/main/java/com/google/api/client/util/FieldInfo.java#L79), 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](https://googleapis.github.io/google-http-java-client/json.html) in particular) with a new paragraph talking about those two annotations.

Thanks.

PS: I think this relates to #127