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
FS_U: Added the command to the docs of SaveData functions
Merge pull request #306 from Subv/even_more_savedata

SaveData: Added some documentation to FormatSaveData