COAP_INVALID_TID: Inform this is deprecated and to use COAP_INVALID_MID

Doxygen comment only, but allow COAP_INVALID_TID to still be used.

include/coap2/pdu.h

@@ -246,6 +246,12 @@ typedef int coap_mid_t;
 /** Indicates an invalid message id. */
 #define COAP_INVALID_MID -1
 
+/**
+ * Indicates an invalid message id.
+ * @deprecated Use COAP_INVALID_MID instead.
+ */
+#define COAP_INVALID_TID COAP_INVALID_MID
+
 /**
  * Indicates that a response is suppressed. This will occur for error
  * responses if the request was received via IP multicast.