Chris Webb shows that comments in the Advanced Editor become step property tooltips:
The June release of Power BI Desktop has what seems to be a fairly unremarkable new feature in that it allows you to add descriptions to each step in a query in the Query Editor window. However the implementation turns out to be a lot more interesting than you might expect: the step descriptions become comments in the M code, and even better if you write M code in the Advanced Editor window your comments appear as descriptions in the Applied Steps pane.
I think this is a smart move, although it does mean that you have to keep those comments up to date…