doc: describe the rollout window behaviour more precisely

Created by: LawnGnome

A support ticket this morning revealed that we probably weren't precise enough in describing how rollout windows operate when a rate limit is in effect, so this is my poorly worded attempt to explain it. Suggestions welcome.

(Since I'm about to go on PTO, I'm totally happy if another batcher wants to merge any good suggestions on wording and merge this PR while I'm out.)

(Also, you have no idea how much it killed me to spell behaviour that way.)