Clarify the keyboard focus warning

The warning is confusing if the developr has set the `tabindex` properly.
I strongly agree on using proper HTML elements, but we still have [another warning](https://github.com/denis-sokolov/REVENGE.CSS/blob/4473d1227a365ed180982937cddb1fd2cf3ed268/revenge.css#L179) that is more specific and clear to developers.