"async & performance": slim down general explanations of iterators/generators

[getify]

Now that "ES6 & Beyond" is going to fully cover iterators and generators, perhaps it'd be appropriate to slim down ch4 a fair bit.

For example, we could strip out some of the nuances around yield *, error propagation, etc.These are all interesting and useful details, but they're not entirely needed to understand using generators for async flow control. I can just point to the "ES6 & Beyond" title for more info.

[andrew-luminal]

Late to the party here, but a huge fan of these books. I agree that this chapter was by far the densest and toughest chapter in the whole series so far, and I'd support slimming it down because it kept me from the really interesting later chapters for a long time.