Update draft-saxe-wimse-token-exchange-and-translation.md

[kenmccracken-google]

Based on suggestions from today's office hours meeting, update the subject_token to require a string constant value. Also fixed grammar and added some MUSTs where appropriate.

[deansaxe]

Relevant notes: https://hackmd.io/@7hs-23SmTxqEA2Gj2l-DIA/Syuh1VBxye

[deansaxe]

This will go into the -02 version when I push it to datatracker.