[w3c/manifest] fix: omit credentials by default when obtaining (#777)

Closes #776 

This change (choose one):

* [ ] Breaks existing normative behavior (please add label "breaking")
* [X] Adds new normative requirements
* [ ] Adds new normative recommendations or optional items
* [ ] Makes only editorial changes (only changes informative sections, or
  changes normative sections without changing behavior)
* [ ] Is a "chore" (metadata, formatting, fixing warnings, etc).

Implementation commitment (delete if not making normative changes):

* [ ] Safari (link to issue)
* [X] Chrome
* [X] Firefox - implemented, link coming soon... 
* [ ] Edge (public signal)

Commit message:
Clarifies that credentials should be omitted by default.  
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/777

-- Commit Summary --

  * fix: omit credentials by default when obtaining

-- File Changes --

    M index.html (17)

-- Patch Links --

https://github.com/w3c/manifest/pull/777.patch
https://github.com/w3c/manifest/pull/777.diff

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