RSC: Add comment explaining use()

[Tobbe]

// `use()` will throw a `SuspenseException` as long as `dataToReturn` is
// unfulfilled. React internally tracks this promise and re-renders this
// component when the promise resolves. When the promise is resolved no
// exception will be thrown and the actual value of the promise will be
// returned instead
// The closest suspense boundary will render its fallback when the
// exception is thrown