SaveData: Added some documentation to FormatSaveData

We still don't know what the other parameters do, but they appear to be very similar to those of FormatThisUserSaveData.
Most likely FormatThisUserSaveData is just an alias for FormatSaveData with LowPathType Empty
Meh, I wouldn't consider it one anymore. Maybe change to "Warning: FormatSaveData parameters ignored" or similar. (If this should be an actual LOG_WARNING is debatable, probably not.)