Clarify issues around optimistic data
This MR:
- Clarifies what we mean by "optimistic data", and removes a qualifications that have not been necessary for a long time.
- Corrects related text about handling unexpected data messages
- Removes "X-Tor-Optimistic" from prop365, since I had added it by false analogy to a corresponding SOCKS feature (which appears not to have ever been implemented)
Closes #349