Improve Documentation - Opaque Containers

Presently, I find the usage of Opaque Containers fromt the documentation hard to understand without an example.

An example here would make it easier for the user to understand, how to use it in code, rather than going to the pyside-setup/sources to find usages of it.

Solution:
Add a short example to create an Opaque Container from a C++ Stl type like std::vector