[ServiceWorker] Complicated sentence is complicated (#773)

This sentence is difficult to grok. Please break it up into multiple sentences:

> A service worker registration has one or more task queues that back up the tasks, whose task source is either the handle fetch task source or the handle functional event task source, from its active worker's event loop's corresponding task queues when the user agent terminates the active worker and re-queue those tasks to the active worker's event loop's corresponding task queues when the active worker spins off. 



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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/773