[ip6] update documentation for otIp6ReceiveCallback (#3584)
diff --git a/include/openthread/ip6.h b/include/openthread/ip6.h
index 30c4af7..26877b7 100644
--- a/include/openthread/ip6.h
+++ b/include/openthread/ip6.h
@@ -413,7 +413,9 @@
/**
* This function pointer is called when an IPv6 datagram is received.
*
- * @param[in] aMessage A pointer to the message buffer containing the received IPv6 datagram.
+ * @param[in] aMessage A pointer to the message buffer containing the received IPv6 datagram. This function transfers
+ * the ownership of the @p aMessage to the receiver of the callback. The message should be
+ * freed by the receiver of the callback after it is processed (@sa otMessageFree).
* @param[in] aContext A pointer to application-specific context.
*
*/