Documentation

Set a private flag "Jismode" to the converter object. There are no reason any code outside libi18n should call this. We are considering move this into intlpriv.h. Don't call this macro unless you are changing libi18n. The name "jismode" refers to the ISO 2022 state (JIS mode). This is what the field was first used for. It is now used for other purposes as well, so the name is no longer appropriate.