Constrain txn-token header field to a single value

WGLC feedback from @bc-pi 

https://datatracker.ietf.org/doc/html/draft-ietf-oauth-transaction-tokens-06#name-txn-token-http-header
The "txn-token" header [field] should maybe also be constrained to a single value and is request only? Maybe some other things too that might be desirable for an HTTP header field definition. YMMV but there will likely be questions with its registration. 
