As Jeffrey said, you should use CKEditor to edit your string variable. Then, in the document template, for that string attribute, set Render XHTML to true.
For this you have to use Rich text widget
You can build a string variable in your microflow with line breaks like this:
- First finalize handling assignment.'
Start with a quote on the first line and write the string on the second line.
Did you try adding line breaks
Try "\r\n" or maybe "<br / >"
EDIT: Haven't tested it
Perhaps you can take a look at the CKEditor widget version 2.2, this version should work with your modeler version.
If you don't want to work with a widget, you have to run the commit through a Microflow and add the linebreaks there.
Good luck, search this forum can also help you find answers, as I believe this is asked before.
<br/> worked for me!