[PR] createContextualFragment returns a DocumentFragment
[PR] createContextualFragment returns a DocumentFragment

Summary:
[Both MDN](https://developer.mozilla.org/en-US/docs/Web/API/Range/createContextualFragment) and the [DOM Parsing and Serialization Working Draft](https://w3c.github.io/DOM-Parsing/#extensions-to-the-range-interface) indicate that this method returns a `DocumentFragment`, not a `Node`.
Pull Request resolved: https://github.com/facebook/flow/pull/7225

Reviewed By: jbrown215

Differential Revision: D13815415

Pulled By: nmote

fbshipit-source-id: 8aeed1c3abdcb5260344f85258d5ddca48fc3a88

popham/flow,gabelevi/flow,facebook/flow,samwgoldman/flow,samwgoldman/flow,samwgoldman/flow,samwgoldman/flow,gabelevi/flow,samwgoldman/flow,popham/flow,popham/flow,gabelevi/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,gabelevi/flow,facebook/flow,facebook/flow,samwgoldman/flow,facebook/flow,samwgoldman/flow,popham/flow,mroch/flow,mroch/flow,mroch/flow,gabelevi/flow,gabelevi/flow,popham/flow,popham/flow,gabelevi/flow,facebook/flow,mroch/flow,popham/flow