Add one more documentation sentence in tcp.c

src/core/tcp.c

@@ -1679,6 +1679,7 @@ tcp_sent(struct tcp_pcb *pcb, tcp_sent_fn sent)
  * @ingroup tcp_raw
  * Used to specify the function that should be called when a fatal error
  * has occurred on the connection.
+ * @note The corresponding pcb is already freed when this callback is called!
  *
  * @param pcb tcp_pcb to set the err callback
  * @param err callback function to call for this pcb when a fatal error