Add key to createPortal reference doc

[mattrunyon]

The createPortal reference page does not mention that you can pass key as the 3rd param to createPortal. This is in both the typescript definitions and here is the implementation

Reconciliation appears to be different if you pass the key to createPortal and not the element passed to createPortal. React dev tools does not seem to show the portal element, so it's not clear in dev tools if you have many sibling portals w/ the key set on the element and not the portal.

My use case where I ran into this was using a layout manager that is not React first, so the React support is kind of an afterthought. I made a change from calling render to createPortal so I could get an array of portals and render them inside my app's React tree and use contexts (render just creates a bunch of sibling root trees).

[gaearon]

Sure, want to add it? I think it's part of the API.

[mattrunyon]

Yep I can make a PR for it