Various small modifications to ZIP 222
Various small modifications to ZIP 222

- Narrow requirements for prefix-free encodings.
- Clarify that TZEs need to define the four properties for each mode.
- Allow for potential future relaxation of cross-TZE restrictions.
- Expand some explanations.

Co-authored-by: Daira Hopwood <cd7d43d5556a38ce626d53826fddc46adc597539@jacaranda.org>

zcash/zips,zcash/zips,zcash/zips,zcash/zips