Add dropdown menu accessibility callout and fix <a> example

Add dropdown menu accessibility callout and fix <a> example

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)