Address post merge PR comments on MRP transmit analytics (#37846)

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Co-authored-by: Boris Zbarsky <bzbarsky@apple.com>

Author: tehampson

src/messaging/ReliableMessageAnalyticsDelegate.h

@@ -68,7 +68,8 @@ class ReliableMessageAnalyticsDelegate
         // this value will be 0.
         uint32_t messageCounter = 0;
         // If the eventType is kRetransmission, this value will be populated with the number of the
-        // retransmission attempt
+        // retransmission attempt. A value of 1 indicates the first retransmission (i.e. the second
+        // transmission of the message). This value should never be 0.
         std::optional<uint8_t> retransmissionCount;
     };