Fix: KeyDown repeats while key is held down (not just KeyPress)

[Copilot]

Docs incorrectly stated that KeyDown fires only once per keypress. In reality, both KeyDown and KeyPress fire repeatedly at the OS repeat rate while a key is held—KeyUp is the only event that fires exactly once.

KeyDown   ← fires
KeyPress  ← fires
KeyDown   ← repeats while held
KeyPress  ← repeats while held
KeyUp     ← fires once on release

Changes

• events.md — Fixed bullet stating "The KeyDown event occurs once"; updated table description to clarify repeat behavior; full style-guide edit pass; date updated to 04/20/2026.
• overview.md — Rewrote the "Order of keyboard events" sequence, which previously implied only KeyPress repeats on hold. Now correctly shows both KeyDown and KeyPress fire on each key-repeat cycle.
• handling-user-input.md — Fixed KeyDown table row from "Raised only when a key is initially pressed" to reflect the repeat behavior.