[ENH] Clarify that "source datasets" do not have to be BIDS, and provide definitions for DOI and URL

[yarikoptic]

came up while reviewing... but could be wrong and we demand only BIDS datasets there?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.71%. Comparing base (df8a853) to head (8f9c9a5).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

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##           master    #2203   +/-   ##
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  Coverage   82.71%   82.71%           
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  Files          20       20           
  Lines        1608     1608           
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  Hits         1330     1330           
  Misses        278      278           

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[effigies]

This seems fine. Duplicating in the DOI and URL fields seems excessive.

[yarikoptic]

I think it is just a result of excessive (if not wrong to start with) specification in both of those. I do not see a need to make them "source dataset specific" and clearly should not relate to "all".