Fix #12563: Elaborate that popover('show') checks title/content

Fixes https://github.com/twbs/bootstrap/issues/12563: Explain in Popover docs that the `show` method will [check](https://github.com/twbs/bootstrap/blob/8ee208e9c197d48828767764d7c89147ebde2a7e/js/tooltip.js#L128) to see if the element's title or content exists before displaying. 

This seemed like the only logical place for it since it should only happen if the `show` method is invokved. 
Another possible place is in the `content` section under Options, but it didn't seem right to explain here that the function would execute twice if `show` is invoked and the [title doesn't exist](https://github.com/twbs/bootstrap/blob/8ee208e9c197d48828767764d7c89147ebde2a7e/js/popover.js#L58-L60).