Missing documentation for Overlay

[edemaine]

There aren't currently any examples of how to use the Overlay component, or a description of what it does, though its props are described in Tooltips and Popovers. By contrast, React Bootstrap has some examples.

I also think target is required and must be an accessor returning an element, contrary to the existing documentation of Overlay's props. Plausibly it should be allowed to be a direct element, not just an accessor. (This is possible with ref if the Overlay comes after the other element.)