Document RelayFeatureFlags

https://github.com/facebook/relay/releases/tag/v10.0.0

> Precise type refinement: gated by RelayFeatureFlags.ENABLE_PRECISE_TYPE_REFINEMENT. With this feature enabled Relay more accurately models GraphQL semantics around inline fragments and fragment spreads which have a different type than their parent selection. This helps Relay to more accurately determine if a query can be fulfilled from the cache (ie with the ‘store-or-network’ fetch policy) and also more accurately determine if data is missing and that a component should suspend. Generally speaking, the change is observable as fewer false positives where Relay thinks data is “missing” when it isn’t. This feature will be enabled by default in a subsequent release; we recommend enabling it now.

How do I enable the suggested feature flag? I am unable to locate any official documented reference to `RelayFeatureFlags`.