[w3c/manifest] Editorial: clarify what document URL is (#959)

Closes #946

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:
Clarify that the document URL is coming from the link element's owner document and that it won't be null.


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<a href="https://pr-preview.s3.amazonaws.com/w3c/manifest/pull/959.html" title="Last updated on Mar 4, 2021, 2:09 AM UTC (448913c)">Preview</a> | <a href="https://pr-preview.s3.amazonaws.com/w3c/manifest/959/2d7bdb9...448913c.html" title="Last updated on Mar 4, 2021, 2:09 AM UTC (448913c)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/959

-- Commit Summary --

  * Editorial: clarify what document URL is

-- File Changes --

    M index.html (7)

-- Patch Links --

https://github.com/w3c/manifest/pull/959.patch
https://github.com/w3c/manifest/pull/959.diff

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