tests: update file's docstring with info about test data

Author: consideRatio

ldapauthenticator/tests/test_ldapauthenticator.py

@@ -1,5 +1,9 @@
-# Inspired by https://github.com/jupyterhub/jupyterhub/blob/main/jupyterhub/tests/test_auth.py
+"""
+Inspired by https://github.com/jupyterhub/jupyterhub/blob/main/jupyterhub/tests/test_auth.py
 
+Testing data is hardcoded in docker-test-openldap, described at
+https://github.com/rroemhild/docker-test-openldap?tab=readme-ov-file#ldap-structure
+"""
 
 async def test_ldap_auth_allowed(authenticator):
     # proper username and password in allowed group