Hello Vlad,
We will improve our documentation.
Wha you need to do is,
Goto UserGroups screen in JoomProfile back-end.
Edit Public usergroup.
and here add the fields, which you want to make it searchable.
Not Searchable usergroups means, if you do not want to allow Public users search for Super User, then select Super User in that field.
I hope it makes sense now.