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).

Doc/library/dl.rst

@@ -31,7 +31,7 @@ libraries. It allows the program to call arbitrary functions in such a library.
 The :mod:`dl` module defines the following function:
 
 
-.. function:: open(name, mode=RTLD_LAZY)
+.. function:: open(name[, mode=RTLD_LAZY])
 
    Open a shared object file, and return a handle. Mode signifies late binding
    (:const:`RTLD_LAZY`) or immediate binding (:const:`RTLD_NOW`). Default is

Doc/library/itertools.rst

@@ -627,7 +627,7 @@ loops that truncate the stream.
                   break
 
 
-.. function:: tee(iterable, n=2)
+.. function:: tee(iterable[, n=2])
 
    Return *n* independent iterators from a single iterable.  Equivalent to::
 

Doc/library/os.rst

@@ -1181,7 +1181,7 @@ Files and Directories
    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

Doc/library/ossaudiodev.rst

@@ -275,7 +275,7 @@ The following convenience methods combine several ioctls, or one ioctl and some
 simple calculations.
 
 
-.. method:: oss_audio_device.setparameters(format, nchannels, samplerate, strict=False)
+.. method:: oss_audio_device.setparameters(format, nchannels, samplerate[, strict=False])
 
    Set the key audio sampling parameters---sample format, number of channels, and
    sampling rate---in one method call.  *format*,  *nchannels*, and *samplerate*