Thats a great idea. It isn't truly a comment, as you indicate with
your quotes, because the query is doing a bit more work and processing
more data. But it's good if you really need it.
I sometimes add a 'comment' when I am building a query particular a tricky
criteria that I want to build in parts.
In the design view I add a new output field like this --
Comment test: "How now brown cow or the quick brown fox jumped"