Clarify pkt-uid field in doxygen

src/common/packet.h

@@ -244,6 +244,16 @@ public:
    * A packet is allocated a new uid when it is created
    * empty or with zero-filled payload.
    *
+   * Note: This uid is an internal uid and cannot be counted on to
+   * provide an accurate counter of how many "simulated packets" of a
+   * particular protocol are in the system. It is not trivial to make
+   * this uid into such a counter, because of questions such as what
+   * should the uid be when the packet is sent over broadcast media, or
+   * when fragmentation occurs. If a user wants to trace actual packet
+   * counts, he or she should look at e.g. the IP ID field or transport
+   * sequence numbers, or other packet or frame counters at other
+   * protocol layers.
+   *
    * \returns an integer identifier which uniquely
    *          identifies this packet.
    */