doc(oneof): mention 'enum'

[Crocmagnon]

Fixes Or Enhances

Mention 'enum' in the documentation for oneof. One of our team member spent too much time looking for an 'enum-like' validation tag, only to find out later that oneof exists. This PR intends on making the discovery of this validation tag easier.

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• Tests exist or have been written that cover this particular change.

@go-playground/validator-maintainers

[Crocmagnon]

Of course I'm open to changing the wording, I'd just like to mention either "enum" or "enumeration" in here to ease discovery.

To add it more smoothly, maybe something like:
"oneof will ensure that the value is one of the values enumerated in the parameter"

[coveralls]

coverage: 74.292%. remained the same
when pulling 97cd405 on Crocmagnon:doc/oneof-enum
into f1939ee on go-playground:master.