[ServiceWorker] Suggested edits to paragraph about declarative-only solutions (#676)

The main change is from "imperative-only" to "declarative-only" in the first sentence, with a few extra alterations as I found the "Having failed serially in many of the same ways" part difficult to follow.

Feel free to rewrite/discard! :)
You can view, comment on, or merge this pull request online at:

  https://github.com/slightlyoff/ServiceWorker/pull/676

-- Commit Summary --

  * Rewrite paragraph about declarative-only solutions

-- File Changes --

    M explainer.md (2)

-- Patch Links --

https://github.com/slightlyoff/ServiceWorker/pull/676.patch
https://github.com/slightlyoff/ServiceWorker/pull/676.diff

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/pull/676