message: Improve comment on element::bool()
diff: @@ -79,7 +79,8 @@ class BSONCXX_API element {
  element();
  ///
  - /// Conversion operator to bool which is true for valid elements.
  + /// Conversion operator to bool which is true for valid elements
  + /// and false for invalid elements.
  ///
  explicit operator bool() const;
  