scrollable-region-focusable docs should mention autofocus scrollable elements in Chrome and Firefox

[straker]

Both Chrome and Firefox automatically add scrollable elements to the tab order (see https://developer.chrome.com/blog/keyboard-focusable-scrollers). However Safari does not do this so not having a tabindex on the element is still an accessibility problem. We should mention this in the help page for scrollable-region-focusable to help users understand what might be happening when their browser is able to focus the element but axe-core reports a violation.