As far as I can tell, the syntax is to return a string. Like

return "This is a string";

The Description attribute is of type large HTML. Technically the content syntax is HTML. So the return value would be "<b>bold text</b>". This will be the content of the description if you boldfaced the text.

However, this won't work. The description will show "<b>bold text</b>". The reason behind this is that the field is internally managed as XML. To set the XML field from a string, there are two methods. One is to create a valid XML from a plain string. This is the one that apparently has been used. The other one is to expect a valid XML and just put it in.

The method that has been chosen changes special characters like < and > into escape sequences. This allows to return something like "1 &lt; 2" without causing an invalid content. Unfortunately the consequence is that you can't really return HTML syntax, because the tags will return with escaped &lt; and &gt; and this results in the result above.

The full HTML syntax that I am aware of is

return "Plain text&lt;br/&gt;&lt;b&gtbold text&lt;/b&gt&lt;br/&gt&lt;i&gtitalic text&lt;/i&gt&lt;br/&gt&lt;a href=”https://jazz.net/forum/”&gt;Forum Link&lt;/a&gt&lt;br/&gtUser link to &lt;b&gt@ralph &lt;/b&gt&lt;br/&gtWork Item link to Defect 3 &lt;br/&gt"

However, this does not work with the method chosen for implementation. Instead

return "Plain text https://jazz.net/forum/”; User link to @ralph;  Work Item link to Defect 3"

will result in a valid link, user mentions and work item link being created. This unfortunately does not allow to create breaks or bold face text however.

The chosen method allows user with almost no experience return their texts and can't corrupt the attribute value, however, It does not allow to use the full HTML syntax.