update pythonization documentation and add C++ callbacks example

[m-fila]

Updated documentation for Python callbacks:

• In the text it was written the pythonization replaces a method, but the code snippet was adding new method and keeping the other. Now the method is replaced in the snippet too.
• fixed formatting of the code snippet. The others snippets are written like interactive session, this one was in a test-style

Added example of C++ callbacks. The example shows replacing methods on an analogous class as in python callback example.

[wlav]

Thanks!