doc/dev/notes: cell-lifecycle: Describe the current state of things.

These are my notes and observations about the circuit and channel reactors. In hindsight, this whole document is just stating the "obvious"¹, so I'm not sure if it's going to be useful for anyone else. Also, since this isn't part of the code docs or tor-proto crate-level docs, it's very likely to become outdated in the near future (LMK if you think any of this should be moved to the tor-proto readme, for example).

At the end of the document, I also wrote a very rough KIST implementation plan based on the discussion from #1712.

Closes #1747 (closed)

(This MR also contains a couple of small doc fixes)

1. scare quotes because the "obvious" is only obvious after you've stared at the circ/chan reactors for a while ↩