Clarify event loop integration and prioritization

[shaseley]

• Reword priority spec section to more closely match the recently added meta explainer
• Add a note in the HTML event loop processing section to describe the intent of the Scheduler priorities wrt event loop integration, along with an example strategy (which ~= what we do in Chromium)

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[mmocny]

Some suggests are attached, but the existing explanations are consistent and accurate, so I leave it to your judgement.

[shaseley]

Thanks @mmocny and @clelland!