Add reusable note to blur, focusout, focusin, focus pages

See https://github.com/jquery/jquery/issues/3123

How about something like this?

Note: Use of focus-related events are complicated by several factors. On Internet Explorer, focus and blur are asynchronous and do not occur at the point where the .focus() method is called in your code. The focusout and focusin events are implemented inconsistently across browsers and not implemented at all on Firefox, although jQuery simulates the event. Code should not depend on the order of focusin and focusout events relative to focus and blur because it varies across browsers. On all platforms, focusout on the previously focused element (if any) will always occur before focusin on the new one, and blur occurs before focus.

Start with the blur, focusout, focusin, and focus API pages in api.jquery.com, and review related issue #3123 for context. Add the reusable focus-event note from this issue to the relevant pages, then verify that the wording and placement are consistent across all four pages.