How to set the public file path to migrate site files and images through Migrate Drupal UI
So it turns out though I had difficulty understanding them, the directions are "correct". The difficulty I had is partly because of my inexperience, but it's also partly because of a slightly confusing UX in Migrate Drupal UI. The label is asking for your "Public files directory", but what it really wants is your absolute docroot path. The directions state this: "To import from you current Drupal Site,enter a local file directory containing your site (e.g /var/www/docroot), or your site address (for example http://example.com)"

This means when the source files UI is asking for the "public files directory", the sites/default/files portion is already assumed by Drupal Migrate and is not needed, nor is it welcome. I used 
 C:\Users\user\Documents\mywebsite and it worked the first try, where repeated attempts that included \sites\default\files path failed. 

Also, the unique settings in settings.php from Acquia dev desktop for the public file path for the new Drupal 8 site were not a factor.  I left those settings as they were by default when I executed the migration.