Clarify that specification text usually starts on the event loop

[domenic]

Closes #2800.

@equalsJeffH, WDYT?

[annevk]

I think this is still a bit confusing since "main thread" is typically used exclusively for the window agents, but not worker agents, and the text you wrote works for both.

[domenic]

In my opinion it's OK, because the term "main thread" only appears once in the intro when trying to make an analogy. Indeed, we can't use that term because it ignores the subtleties of the situation, but that's kind of what that paragraph is trying to point out.