Clarified DID fields

[Edmund Jay]

Comments from PD regarding for pull request #39

https://bitbucket.org/openid/connect/pull-requests/39/merging-cp-into-ca#comment-237220247

There appear to be two did fields identified in this spec - one in the object, and one in the authorization request query parameter. Suggest we make it very clear whether the steps in this section apply to one, the other, or both.

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