Clarify what is stored on the ledger

[SteveLasker]

A reader of the Arch doc may assume the content of the signed statements is stored on the Ledger, as opposed to verifiable proofs of the signed statement.
This has evolved over time as, at least some of us, assumed the signed statement was stored on the ledger.
The current belief of the authors is the ledger stores "hashes", verifiable proofs of the signed statement, that can live forever without concerns of PII information. However, the current state of the doc doesn't really qualify this distinction.

Storing the actual signed statement, including the payload, or other elements of the protected header on the ledger also raises PII information concerns, such as #322

A complete Transparency Service may provide ancillary services that may store the content of the signed statement, or even the contents of a hashed payload, however that's beyond the scope of the SCITT Draft.

This issue tracks:

• Clarifying the ledger only stores "proofs" (or a better agreed upon term)
• Clarifying a service may store the signed statements, but likely not on the ledger so they can delete and/or redact PII information that may be written (intentionally or accidentally)