Added documentation and annotation to enum variants

[ko1N]

This pull request enables enum variants to also have doc comments and annotations much like fields.

[neysofu]

This looks good, but I'm not so sure about exposing Variant internals with pub. Any specific reason why you would like to do that?

[ko1N]

I required this when generating Serde models from a Json specification. In particular I needed to generate the Serde rename annotations for Enum variants.

[charnz]

I have a similar use-case (generating Serde models) and would be keen to see this merged. Looking at the Field struct it seems that pub is used for the members there so its not without precedent. Only thing I can think of is i tmight be good to have a test for this similar to single_struct_documented_field for Field?