Document fetchPolicy option for fetchQuery

[skrabe]

Follow-up to #4880.

The fetchPolicy option isn't mentioned in the fetchQuery docs, which is what caused the confusion in that issue. This adds it to the Arguments section with the two supported values (network-only, store-or-network) and an explicit note that store-and-network and store-only aren't supported, pointing people at useLazyLoadQuery instead.

The TypeScript types on DefinitelyTyped already match the Flow type (FetchQueryFetchPolicy = "store-or-network" | "network-only"), so nothing was needed on that side.

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