R4 needs more discussion of user-triggered events

[pprindeville]

The "Working with events" sections mentions user-triggered events, but then doesn't discuss a canonical way of having user-triggered events.

Can this be added?

[nmathewson]

I'd love to, but I'm not working on libevent much right now. If anybody else has time to write this up, I'd be happy to take a good pull request.