[ServiceWorker] ServiceWorker scope documentation in explainer.md (#797)

The current documentation is a little misleading – it appears to say the default scope is the web root. 
In reality the default scope is relative to the location of the ServiceWorker script.
If a ServiceWorker is located in: `/my-app/sw.js` and the scope is set to: `scope: '/'` an error will throw:

`DOMException: Failed to register a ServiceWorker: The path of the provided scope ('/') is not under the max scope allowed ('/assets/'). Adjust the scope, move the Service Worker script, or use the Service-Worker-Allowed HTTP header to allow the scope.`

I've updated the documentation to show this.
You can view, comment on, or merge this pull request online at:

  https://github.com/slightlyoff/ServiceWorker/pull/797

-- Commit Summary --

  * ServiceWorker scope documentation in explainer.md

-- File Changes --

    M explainer.md (2)

-- Patch Links --

https://github.com/slightlyoff/ServiceWorker/pull/797.patch
https://github.com/slightlyoff/ServiceWorker/pull/797.diff

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/pull/797