Created by: blankmaker
Fixes https://github.com/twbs/bootstrap/issues/12563: Explain in Popover docs that the show method will check 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.