Restore [] where default arguments are not keywords
Reverts some changes of d13fdd97cc8e.
diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 8b2e582..3f43338 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -1145,7 +1145,7 @@
Availability: Unix.
-.. function:: mknod(filename, mode=0o600, device=0)
+.. function:: mknod(filename[, mode=0o600[, device=0]])
Create a filesystem node (file, device special file or named pipe) named
*filename*. *mode* specifies both the permissions to use and the type of node