add Doc comment explaining copy behaviour

include/CAN.hpp

@@ -33,6 +33,15 @@ class CAN {
     close(sock);
   }
 
+  /**
+   * Duplicate the CAN socket.
+   * 
+   * IMPORTANT NOTE: Frames are not replicated between CAN objects!
+   * All copies of a CAN object share the same kernel buffers, so only one copy 
+   * of CAN will receive any given frame. Think of it like sharing the socket
+   * but C++/the compiler and the kernel handle thread safety and
+   * resource management.
+   */
   CAN(const CAN& s) {
     sock = dup(s.sock);
   }