Replacement token `sub` and `aud` modification language

[tulshi]

There was a point in Yaron's feedback email about specifying in 7.4.1 that sub must be unchanged, and although this is mentioned in 7.4, perhaps we should add bullet points in 7.4.1 (Txn-Token Service Responsibilities) that specify that sub MUST NOT be modified and aud MUST NOT be modified.