Add documentation around our focus rings for buttons changing from focus to focus-visible

[mannycarrera4]

💡 Suggestion

It's unclear the change we made when going from focus to focus-visible in out buttons. Consumers where expecting a focus ring when using the mouse, especially around Popups/Modals. However, because we changed our buttons to use focus-visible we should maybe document this either in the v10 migration guide or our button documentation.

MDN docs