Add missing docstring

Author: Sporiff

src/main/java/org/openpodcastapi/opa/subscription/service/UserSubscriptionService.java

@@ -128,6 +128,7 @@ public BulkSubscriptionResponse addSubscriptions(List<SubscriptionCreateDto> req
     ///
     /// @param feedUUID the UUID of the subscription feed
     /// @param userId   the ID of the user
+    /// @return a [UserSubscriptionDto] containing the updated object
     @Transactional
     public UserSubscriptionDto unsubscribeUserFromFeed(UUID feedUUID, Long userId) {
         UserSubscription subscription = userSubscriptionRepository.findByUserIdAndSubscriptionUuid(userId, feedUUID)