Some def. queries can be quite long and complicated. That's why a description box right under the query in the layer's Properties box is needed where we could leave our notes in plain English.
Although I do agree with your suggestion, you can use SQL comments in the DQ. The separate box you are suggesting would have to be kept up to date with the SQL, otherwise it would do more harm than good.
Another consideration is who looks at the DQ tab. If the DQ is permanent, it should probably be commented in the General tab of the layer. I do this for layers.
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