Clearer documentation for dataget

[alyssadai]

Two additional pieces of info I think would be helpful for a dataget user in the "How to get data" modal:

1. Look for Finished getting all files for the matching subject(s)/session(s) from DataLad to indicate the data fetching is complete. Otherwise, due to the verbosity of dataget logs (see related Reduce verbosity and include progress counter dataget#17), it's hard to tell when DataLad is hanging or just taking a long time for a bigger dataset.

   e.g., you'll often see the following which looks deceptively like a completion message:

   action summary:
     get (ok: 7)
   

2. A brief explainer that dataget will 'clone' the entire repo for a matching dataset, but only get the contents of specific matching subjects within that repo. While we refer to "matching subjects" in the modal, I think it's not obvious to users unfamiliar with DataLad how our method differs from a regular git clone or just downloading the entire dataset (esp because if you look at the output directory contents, it appears as if all the subjects and subject data is there).