Added documentation section explaining how to use Callable provider i…

[TomaszBorczyk]

This is a followup to the discussion here: https://github.com/ets-labs/python-dependency-injector/issues/704

[zulrang]

This needs to be merged in. I spent over an hour trying to make this work.

[TomaszBorczyk]

@zulrang would love to, but I don't have write access to master. Seems like master branch hasn't been updated for months, the project seems stale from the perspective of maintainers activity. I could open PR to develop, but that would not update the documentation :/