binary input format: clarify size of header fields in documentation
binary input format: clarify size of header fields in documentation

ihc/futhark,diku-dk/futhark,ihc/futhark,ihc/futhark,diku-dk/futhark,HIPERFIT/futhark,HIPERFIT/futhark,diku-dk/futhark,diku-dk/futhark,HIPERFIT/futhark,diku-dk/futhark