Add doctest info in testingmdx
# What does this PR do?

Finding information on how to properly test the docstring is currently pretty hard! As far as I know, the only good explanation is in `transformers/utils/prepare_for_doc_test.py` but it does not appear in the doc. I hope this will help people debug their docstring examples. 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Yes, gonna clean the github history 