#14804: Remove [] around optional arguments with default values

Mostly just mechanical removal of []. In some rare cases I've pulled the
default value up into the argument list.
diff --git a/Doc/library/itertools.rst b/Doc/library/itertools.rst
index a553d09..477ea86 100644
--- a/Doc/library/itertools.rst
+++ b/Doc/library/itertools.rst
@@ -627,7 +627,7 @@
                   break
 
 
-.. function:: tee(iterable[, n=2])
+.. function:: tee(iterable, n=2)
 
    Return *n* independent iterators from a single iterable.  Equivalent to::