As the question often comes up about why Bootstrap does not do "true"
ARIA menus (with their role="menu" etc), add an admittedly lengthy
note (tl;dr because BS is generic, and ARIA menus are specific and quite
limiting).
Additionally, fixes up the <a> example for dropdown trigger, with the
missing role="button" and neutering the href (which would be useless
anyway since BS overrides the link-like nature of the <a> so it could
never be triggered/followed anyway)