Attestations: document data, claim type, and signature properties

[tyleryasaka]

We should clarify the significance of each of these properties. For reference, these are specified as part of ERC735.

We should keep in mind, and probably note in the docs, that an understanding of these properties is not required in order to use the API. They are simply used for validation and origin-js does all the validation behind the scenes. (Invalid attestations will never show up in the user object that is returned from the get method.) Our explanation should be provided simply to satisfy the curiosity of developers and provide transparency in our implementation. 😃