[disk] Add comment to disk.Usage().

disk/disk_unix.go

@@ -4,6 +4,9 @@ package disk
 
 import "golang.org/x/sys/unix"
 
+// Usage returns a file system usage. path is a filessytem path such
+// as "/", not device file path like "/dev/vda1".  If you want to use
+// a return value of disk.Partitions, use "Mountpoint" not "Device".
 func Usage(path string) (*UsageStat, error) {
 	stat := unix.Statfs_t{}
 	err := unix.Statfs(path, &stat)