Docs: responsive tables reflow needs some explanatory text

http://v4-alpha.getbootstrap.com/content/tables/#reflow

Needs some general introduction of what reflow actually does, as well as some strong-ish warning that this is only applicable to a certain type / arrangement of content (where the first "column" is actually the <thead>, and the other columns contain data...so effectively, a traditional table turned on its side). If devs blindly take this table and stuff their "regular" table content in there, it may lead to nonsensical structures.