[doc] Coin: explain that IsSpent() can also mean never existed

src/coins.h

@@ -73,6 +73,9 @@ public:
         ::Unserialize(s, Using<TxOutCompression>(out));
     }
 
+    /** Either this coin never existed (see e.g. coinEmpty in coins.cpp), or it
+      * did exist and has been spent.
+      */
     bool IsSpent() const {
         return out.IsNull();
     }