Modify documentation of cmd.Cmd to indicate subtle behaviours

[pythonhacker]

Documentation

This is w.r.t the issue #143804

It was pointed out it is not a bug (agreed) but we can improve the documentation of this module
since it is now not very clear that

• One has to over-ride "emptyline" so that the last line is not repeated
• One has to over-ride do_EOF to quit cleanly

A tutorial on this in the documentation would be helpful as right now these are at the level of method level documentation and quite unclear at the usage level.

[sabasiddique1]

I’d like to work on #144338. I can add a short example in cmd docs showing how to override emptyline() and do_EOF() for shells that use default().