Clean up keymint ProtectedData documentation

- Fix up some minor CDDL formatting issues.
- Add more definition around the BCC, hopefully clearing up partner
  confusion around how to implement it.
- Explain when BccPayload entries may be omitted in the case of a
  "Degenerate BCC"
- Add a bit more description to the DKSignature format

Bug: 227350250
Test: N/A -- doc changes only
Change-Id: I28337a80e2b49661cc37876400d7ac3b8759ba01