[DOCS] Explain better the concept of Esc as a modifier

[icalvo]

Using <Esc> as a modifier key is a very unusual thing in my experience. When I read about the mode transitions I didn't understand it and I rather tried them as a key combo (like <Esc>, release, and then an arrow key).

The idea is fine, but I think it should be explained clearly in the Usage docs, maybe even highlight it with an info box.

[BenjaminKobjolke]

Indeed, I had the same problem