Re: [w3ctag/spec-reviews] Performance Observer Review (formerly Navigation Timing 2 concepts) (#18)

Apologies for the delay in getting back to everyone. Overall [Performance Timeline 2](https://www.w3.org/TR/performance-timeline-2/) looks good!

A few things stand out (some of them are editorial, sorry!):

 - It'd be great for there to be a separate explainer doc that goes into more detail about the problems that NT2 solves and the sample code you might write to use it to address those issues. 
 - In section 3, step 3.1, where is `new entry` allocated?
 - In step 3, are you _really_ meaning to get the list of registered performance observer objects from the global environment? Could it be a more standard data structure like a list or map hidden away using a symbol?
 - The "low priority queue" language is interesting...are there other specs that use similar priority statements that you're aware of?
 - Where is the list of types that you can listen for via a PerformanceObserver listed? E.g., `frame` seems to relate to [Frame Timing](https://www.w3.org/TR/frame-timing/), but I'm not sure what's definitive.
 - Can Performance Observers be used to observe page load that may have already happened? How does that work?

Thanks!

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