Restore [] where default arguments are not keywords
Reverts some changes of a36666c52115.
In the case of os's `mknod(filename[, mode=0600[, device=0]])`, I have nested
the [] as setting mode doesn't require to set device (but setting device
requires to set mode).
diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 8569f82..025042a 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -1181,7 +1181,7 @@
doesn't open the FIFO --- it just creates the rendezvous point.
-.. function:: mknod(filename, mode=0600, device=0)
+.. function:: mknod(filename[, mode=0600[, 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 to