propagation: clarify Cost231 frequency validity and default usage

propagation: simplify Cost231 Frequency attribute documentation

propagation: simplify Cost231 Frequency attribute documentation

Author: animeshsri14

src/propagation/model/cost231-propagation-loss-model.cc

@@ -39,7 +39,7 @@ Cost231PropagationLossModel::GetTypeId()
                           MakeDoubleAccessor(&Cost231PropagationLossModel::m_lambda),
                           MakeDoubleChecker<double>())
             .AddAttribute("Frequency",
-                          "The Frequency  (default is 2.3 GHz).",
+                          "Carrier Frequency.",
                           DoubleValue(2.3e9),
                           MakeDoubleAccessor(&Cost231PropagationLossModel::m_frequency),
                           MakeDoubleChecker<double>())

src/propagation/model/cost231-propagation-loss-model.h

@@ -35,6 +35,12 @@ namespace ns3
  *  Base station Antenna Height: 30m to 200m
  *  Link Distance:up to 20 km
  *
+ *  @note Although the classical COST-231 model has been empirically validated
+ *  in the 1500 MHz to 2000 MHz frequency range, the ns-3 implementation
+ *  historically defaults to 2.3 GHz due to its original use in WiMAX studies.
+ *  Users are advised to consciously select the carrier frequency appropriate
+ *  for their scenario and to exercise caution when operating far outside the
+ *  classical validity range.
  */
 
 class Cost231PropagationLossModel : public PropagationLossModel