Explain that the assignee is not necessarily the reviewer

[oli-obk]

Also give "one week" as the timeframe within any reaction (reassignment, "will take longer to review", or a review) will happen. Happy to increase the time here.

Related discussion: https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/review.20queue.20.26.20capacity

cc @rust-lang/compiler

[estebank]

@bors r+

Edit: Ah, we don't use triage bot for the triage bot repo 🤦

[jackh726]

One week is pretty quick? Don't we normally say 2 weeks?