DOC Mention estimator_checks.py under Common Tests

Author: jnothman

doc/glossary.rst

@@ -183,7 +183,8 @@ General Concepts
         This refers to the tests run on almost every estimator class in
         Scikit-learn to check they comply with basic API conventions.  They are
         available for external use through
-        :func:`utils.estimator_checks.check_estimator`.
+        :func:`utils.estimator_checks.check_estimator`, with most of the
+        implementation in ``sklearn/utils/estimator_checks.py``.
 
         Note: Some exceptions to the common testing regime are currently
         hard-coded into the library, but we hope to replace this by marking