[w3c/manifest] Editorial: add better example of scope (#958)

Closes #380 

This change (choose at least one, delete ones that don't apply):

* Makes editorial changes (changes informative sections, or changes normative sections without changing behavior)

Commit message:
Clarifies the how scope works, how it relates to the manifest being "applied",  and limitations.


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<a href="https://pr-preview.s3.amazonaws.com/w3c/manifest/pull/958.html" title="Last updated on Mar 4, 2021, 1:26 AM UTC (79b05e3)">Preview</a> | <a href="https://pr-preview.s3.amazonaws.com/w3c/manifest/958/2d7bdb9...79b05e3.html" title="Last updated on Mar 4, 2021, 1:26 AM UTC (79b05e3)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/958

-- Commit Summary --

  * Editorial: add better example of scope

-- File Changes --

    M index.html (58)

-- Patch Links --

https://github.com/w3c/manifest/pull/958.patch
https://github.com/w3c/manifest/pull/958.diff

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