bpo-46196: document method cmd.Cmd.columnize (#30303)

The method is already written and tested, now it's officially public.

Doc/library/cmd.rst

@@ -121,6 +121,13 @@ A :class:`Cmd` instance has the following methods:
    :meth:`complete_\*` method is available.  By default, it returns an empty list.
 
 
+.. method:: Cmd.columnize(list, displaywidth=80)
+
+   Method called to display a list of strings as a compact set of columns.
+   Each column is only as wide as necessary.
+   Columns are separated by two spaces for readability.
+
+
 .. method:: Cmd.precmd(line)
 
    Hook method executed just before the command line *line* is interpreted, but

Misc/NEWS.d/next/Documentation/2021-12-30-19-12-24.bpo-46196.UvQ8Sq.rst

@@ -0,0 +1 @@
+Document method :meth:`cmd.Cmd.columnize`.