It has been pointed out to me that this box is not really for a summary, but rather for the reason for your edit.
But these are the same thing. When you write a "summary" of something, you explain, using usually all-new wording and certainly much more tersely, the essence of what it is you are summarizing. In this case, you're supposed to be writing a summary of your "edit". An edit is not, in its essentials, really about the +two characters here, -5 characters there minutia. And the easiest and best way to see those is almost always in one of the diff views. No, an edit is about making changes so the post will have certain desirable characteristics. And once you list the desirable characteristics that you had to edit to meet, or the types of changes needed to meet those characteristics, you have an itemized edit rationale ready to go.
Of course, most people are very bad at writing titles, summaries, and so forth. This is a difficult task and not a skill most practice very much. But I'm not convinced trying to fit in two or three more words, or even a line or two of just-in-time help, will make any real difference. This is a subject for a college course, in all likelihood.