ENH: Clarify that "raw dataset" source data in example layout is a raw BIDS dataset

[yarikoptic]

and also did not include under "raw" subfolder

attn @nikhil153 since relates to a bit more "involved" discussion/approach of

• [WIP] Suggested modifications to directory layout of the bids-study DatasetType #2191

edit: continuing on above relation to #2191 -- this + 1 more spot is where might be worth changing "raw" into "bids-raw" or "raw-bids" (under sourcedata/) to signal that it is not just "raw" recording of some kind, but rather a raw BIDS dataset.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.71%. Comparing base (73281ae) to head (3828d79).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2210   +/-   ##
=======================================
  Coverage   82.71%   82.71%           
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  Files          20       20           
  Lines        1608     1608           
=======================================
  Hits         1330     1330           
  Misses        278      278           

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