Consider making .tooltip('show') throw an error when the target is `display: none`

This would allow us to drop a callout from the tooltip & popover docs and make the mistake more obvious to our users.

The current callout:

> #### Don't try to show tooltips on hidden elements
> 
> Invoking `$(...).tooltip('show')` when the target element is `display: none;` will cause the tooltip to be incorrectly positioned.
