RFA: add example to docs for python function

[Eli Zaretskii]

> From: Tom Tromey <tromey@redhat.com>
> Date: Mon, 05 Nov 2012 10:54:53 -0700
> 
> PR 14802 concerns a users confusion about the purpose of gdb.Function.
> After reading the PR I thought that an additional example showing how to
> use the resulting function would avoid any future confusion.
> 
> Ok?

Fine with me, thanks.