Add some additional documentation to the file format code.
Add some additional documentation to the file format code.

This should be helpful to users writing file format add-ins.

Fenex/Pinta,PintaProject/Pinta,PintaProject/Pinta,Mailaender/Pinta,Fenex/Pinta,Mailaender/Pinta,jakeclawson/Pinta,PintaProject/Pinta,jakeclawson/Pinta