lrwpan: clarify the role of U/L bit in the interface identifier.

src/lr-wpan/model/lr-wpan-net-device.h

@@ -191,9 +191,18 @@ private:
   void CompleteConfig (void);
 
   /**
-   * Builds a Mac48Address from the PanId and Short Address
+   * Builds a "pseudo 48-bit address" from the PanId and Short Address
    * The form is PanId : 0x0 : 0x0 : ShortAddress
    *
+   * The address follows RFC 4944, section 6, and it is used to build an
+   * Interface ID.
+   *
+   * The Interface ID should have its U/L bit is set to zero, to indicate that
+   * this interface ID is not globally unique.
+   * However, the U/L bit flipping is performed when the IPv6 address is created.
+   *
+   * As a consequence, here we set it to 1.
+   *
    * \param panId The PanID
    * \param shortMac The Short MAC address
    * \return a Pseudo-Mac48Adress