wifi: Better document that GetMcsValue() cannot be called for non-HT

src/wifi/model/wifi-mode.cc

@@ -157,8 +157,7 @@ WifiMode::GetMcsValue() const
     }
     else
     {
-        // We should not go here!
-        NS_ASSERT(false);
+        NS_ASSERT_MSG(false, "GetMcsValue() cannot be called for non-HT modulations");
         return 0;
     }
 }

src/wifi/model/wifi-mode.h

@@ -113,6 +113,9 @@ class WifiMode
     uint16_t GetConstellationSize() const;
     /**
      * @returns the MCS value.
+     *
+     * This method cannot be called for non-HT modulations, because Modulation and Coding Scheme
+     * (MCS) is a concept introduced by the 802.11n amendment.
      */
     uint8_t GetMcsValue() const;
     /**