docs: getPayout() throws if there's no payment for the keyword

Author: Chris-Hibbert

main/reference/zoe-api/user-seat.md

@@ -66,6 +66,9 @@ redirecting escrowed assets in accordance with the result of the transaction.
 
 The promise will be resolved promptly once the seat exits.
 
+If there is no payment corresponding to the keyword, an error will be
+thrown.
+
 ## E(UserSeat).getOfferResult()
 
 - Returns: **Promise<OfferResult>**