Text formatting for strings in Google Documents from Google Apps Script

Text formatting for strings in Google Documents from Google Apps Script

I have some Google Apps scripts that use the DocumentApp .replaceText
method to find a text string in a Google Document and replace it with a
different text string.
var file = DocsList.getFileById('xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx');
var newTextString = 'foo\nbar';
file.replaceText('Old text string',newTextString);
The \n works to make line breaks, but is there a way to format the text
with bold/italics/colors/etc?
I tried to use HTML tags and inline CSS styles with the span tag, but that
didn't work, it just outputs the text including the tags. I remember how
Google Docs used to have an HTML button and you could see the markup that
made up the content of the Google doc, but they took that out.