[C++][FlightRPC] Document StatementAttributeId enum values in ODBC SPI

### Describe the enhancement requested

https://github.com/apache/arrow/commit/ed36107ad8#diff-cbd67e664d899cd38324e1b1a05b073722bd9c011a3449127c408efcf2f1592eR42-R52

There is a todo in https://github.com/apache/arrow/commit/ed36107ad8 which I presume that the author just forgot to do it.

### Component(s)

C++