Clarify a Service Worker's scriptURL is used in get/getAll

[aamuley]

Changes the note above the get and getAll method steps to from a Service Workers "registration scope" to its script url since that is more well-defined for SWs. Also removes the carveout for Window objects since a SW's API base URL is defined as the script URL as well.
Fixes #247

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[DCtheTall]

Change LGTM, but I think the title of the PR might need rethinking, I think it is sort of misleading/confusing to say the registration's "scope" is its script URL, when scope is already a defined term wrt registrations with a particular meaning (and it is usually not the script URL, since it defaults to script's origin).

[aamuley]

ahh yep i see, tried to make that clearer but open to further wording suggestions