Trust Mark Claim Clarification

[MichaelFraser1999]

As it stands, the wording on the trust mark claims iss and sub is ambiguous - no text explicitly says if these should be Entity Identifiers or not. I believe this should be explicitly clarified

[MichaelFraser1999]

Pull request created with proposed text. Aligns the definitions of the fields to how the same fields are defined for Entity Statements