A couple of things:
have the option to provide "diversity statements" or "personal statements"
Usually, a personal statement is required along with a "research statement." Often, a diversity statement can be supplied if you want room to talk about your unique experiences. Increasingly, it is required. You don't have to provide one, if you don't want.
As a foreigner, I can't seem to grasp what I should write about...
I'm not sure if this is sarcasm, but the point of a diversity statement is to explain how your experiences are different from the majority of people in the field. For physics in the US, this is white men, equally distributed between US nationals and visa holders (warning: PDFs). Often, these statements contextualize your experience to people who lived a different life from you. But they can also explain how you'll bring a different perspective to your field of study. I don't know exactly what that might look like in physics, but different perspectives are needed in fields like psychology, or image processing.
But diversity doesn't need to be race or gender. Parents of people with PhDs are way more educated (PDF) than the average American. So one could write about how growing up with a less educated family affected your approach to education.
To your specific question, writing about your ADHD (especially in Korean culture) would be totally appropriate. It doesn't have to be woe-is-me, but it serves to contextualize to your experience. Presented with two equally qualified candidates, one having worked through ADHD might be a stronger candidate for graduate school (data).
Also consider the university environment. If classes, the program, etc, have all been designed around people without attention deficits, having more people with ADHD join the program/university will force them to improve their practices. (You don't have to be the guinea pig for this, either.)
Tl;dr: You don't have to write about your ADHD (or anything), but if you think you can answer the prompt appropriately with it (you didn't share the prompt, so I can't say), then it would be a good choice IMO. But I also agree that it's unlikely to make or break your application.