Write about `@oneOf` in the graphql-js documentation (#4290)
Currently input-unions and by extension the `@oneOf` directive aren't
present in the documentation. I have opted to put this into the advanced
section. The copy might be up for improvement, honestly fire away if
there's more cases to cover, just wanted to get the ball rolling here.
CC @benjie
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Co-authored-by: Benjie <benjie@jemjie.com>
