I'd agree that there is flakiness with trying to keep the formatting for description of tools. What I found to semi-works lately, is to use html code in the Tool parameter description box ... e.g <br><br> and <u><li> etc., the use the Description tab in catalog....and go back and forth. Kind of a pain, but I have been able to get the formatting, including lists and text formatting to stick. There may be other ways like starting int he Description (as the link Dan sent shows), but most of mine already had some description from the tool box.
Sample...but some html code in the toolbox
look at it in Description tab, doesn't look right (results var)
go into EDIT from the descirption
Save, with no other changes
The sidebar help in the tool
Sometimes I can continue editing in the Description box and have the formatting stick...sometimes I have to go back, add a little more html tags for wha tI'm trying to do, and then go edit in the description key again, and it will work.....and may need to inthis case because it now looks like it has an extra <br> Not a smooth process, and again, maybe starting in the Description tab to begin with might help. I haven't tested fully.