Respond to CR.

// FREEBIE

SignalServiceKit/src/Messages/DeviceSyncing/OWSOutgoingSentMessageTranscript.m

@@ -13,6 +13,9 @@ NS_ASSUME_NONNULL_BEGIN
 /**
  * Normally this is private, but we need to embed this
  * data structure within our own.
+ *
+ * recipientId is nil when building "sent" sync messages for messages
+ * sent to groups.
  */
 - (OWSSignalServiceProtosDataMessage *)buildDataMessage:(NSString *_Nullable)recipientId;
 

SignalServiceKit/src/Messages/Interactions/TSOutgoingMessage.m

@@ -455,6 +455,8 @@ NSString *const kTSOutgoingMessageSentRecipientAll = @"kTSOutgoingMessageSentRec
     return builder;
 }
 
+// recipientId is nil when building "sent" sync messages for messages
+// sent to groups.
 - (OWSSignalServiceProtosDataMessage *)buildDataMessage:(NSString *_Nullable)recipientId
 {
     OWSAssert(self.thread);