Pitch: Add an Actor Reentrancy section to TSPL

Hi all — I'm Alex Apostol, currently in the Swift Mentorship Program, mentored by @maartene. As my mentorship task, I'm working through the open-source contribution flow for Swift, and Maarten and I landed on documentation as a good first target. While reviewing the Concurrency chapter, we noticed TSPL doesn't currently document actor reentrancy — the fact that an actor can be reentered by another call while a previous call is suspended at a potential suspension point. Reentrancy is a deliberat...