fix: [Federation] Trust marks explanatory text

[Giuseppe De Marco]

Some OP implementers pointed out that the explanatory text related to the Trust Mark JSON object text may have bad interpretations and asked more information to get sure on the final result.

thy asked if the id claim in the json object SHOULD or MUST be the same of the Trust Marks JWT, the answer is yes and it must be explicit in the text.

This make me though and I decided to push the following PR to fix this issue
https://bitbucket.org/openid/connect/pull-requests/343

[Giuseppe De Marco]

Closed via https://bitbucket.org/openid/connect/pull-requests/343