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.
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.
For this you have to use Rich text widget
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.
Did you try adding line breaks
Try "\r\n" or maybe "<br / >"
EDIT: Haven't tested it