[manifest] expanded navigation scope text (#296)

* added how to check if URL is within scope 
* defined unbounded - for when there is no scope.
* added some security considerations
You can merge this Pull Request by running:

  git pull https://github.com/w3c/manifest scope_expansion

Or you can view, comment on it, or merge it online at:

  https://github.com/w3c/manifest/pull/296

-- Commit Summary --

  * expanded navigation scope text

-- File Changes --

    M index.html (68)

-- Patch Links --

https://github.com/w3c/manifest/pull/296.patch
https://github.com/w3c/manifest/pull/296.diff

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/pull/296