um: document arch_futex_atomic_op_inuser

arch_futex_atomic_op_inuser was not documented correctly
resulting in build time warnings.
Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/kernel/skas/uaccess.c

@@ -236,7 +236,9 @@ EXPORT_SYMBOL(strnlen_user);
  *			  argument and comparison of the previous
  *			  futex value with another constant.
  *
- * @encoded_op:	encoded operation to execute
+ * @op:		operation to execute
+ * @oparg:	argument to operation
+ * @oval:	old value at uaddr
  * @uaddr:	pointer to user space address
  *
  * Return: