[w3c/manifest] Editorial: clarify where to get apps name from (#927)

Closes #622 

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:
Makes is more clear where the name can be derived from: from user, from the OS, from document, etc.


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<a href="https://pr-preview.s3.amazonaws.com/w3c/manifest/pull/927.html" title="Last updated on Jul 20, 2020, 8:27 AM UTC (42d1021)">Preview</a> | <a href="https://pr-preview.s3.amazonaws.com/w3c/manifest/927/96246fb...42d1021.html" title="Last updated on Jul 20, 2020, 8:27 AM UTC (42d1021)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/927

-- Commit Summary --

  * Editorial: clarify where to get apps name from

-- File Changes --

    M index.html (27)

-- Patch Links --

https://github.com/w3c/manifest/pull/927.patch
https://github.com/w3c/manifest/pull/927.diff

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