message: [doc] Coin: explain that IsSpent() can also mean never existed
diff: @@ -100,6 +100,9 @@ public:
  ::Unserialize(s, VARINT(nTime));
  }
  + /** 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();
  }
  