Revise and add additional 0-rtt doc comments

[gretchenfrage]

I had a hard time getting 0-RTT actually working in practice. It escalated to me forking rustls so I could put in a bunch of print statements. Although I did find the 0-rtt tests, there were some subtle enough pitfalls that I didn't notice the relevant ways my attempts were diverging.

In hopes of improving understandability and discoverability of this, this PR adds notes about these pitfalls to some of the doc comments, and also largely rewrites the doc comment for Connecting::into_0rtt to make its behavior more clear, especially across various edge cases.

This PR does not change any code or APIs.