gh-58083: Expand the introduction of the cmd module (#155738)

Co-authored-by: Stan Ulbrych <stan@python.org>

Author: mangrisano

Doc/library/cmd.rst

@@ -13,6 +13,13 @@ command interpreters.  These are often useful for test harnesses, administrative
 tools, and prototypes that will later be wrapped in a more sophisticated
 interface.
 
+A command interpreter built with :class:`Cmd` typically presents the user
+with an interactive prompt, reads input one line at a time, and dispatches each
+command to a method named ``do_`` followed by the command name.  For example,
+the ``help`` command is handled by the :meth:`~Cmd.do_help` method.  Apart from
+the built-in ``help`` command, most of the interpreter's behavior is defined by
+the ``do_*`` methods you add in your own subclass.
+
 .. class:: Cmd(completekey='tab', stdin=None, stdout=None)
 
    A :class:`Cmd` instance or subclass instance is a line-oriented interpreter