Add some documentation to pkg/system

Partially addresses #11581

Signed-off-by: Ankush Agarwal <ankushagarwal11@gmail.com>

system/mknod.go

@@ -6,6 +6,8 @@ import (
 	"syscall"
 )
 
+// Mknod creates a filesystem node (file, device special file or named pipe) named path
+// with attributes specified by mode and dev
 func Mknod(path string, mode uint32, dev int) error {
 	return syscall.Mknod(path, mode, dev)
 }