add docstring to explain new function

Author: Christina Koss

rasa/test.py

@@ -33,6 +33,21 @@ def test_core_models_in_directory(
 
 
 def _get_sanitized_model_directory(model_directory: Text) -> Text:
+    """Adjusts the '--model' argument of 'rasa test core' when called with '--evaluate-model-directory'.
+
+    If a file is passed as 'model_directory' to 'test_core_models_in_directory' instead of a directory, returns the
+    directory that file is contained in. Prints a warning if the file is not the latest model. If the passed
+    'model_directory' is the latest model, it's probably caused by the default value of the CLI. Doesn't warn in this
+    case.
+    If anything other than an existing file is passed as 'model_directory' to 'test_core_models_in, returns the
+    'model_directory' argument as is.
+
+    Args:
+        model_directory: the 'model_directory' argument that was given to 'test_core_models_in_directory'
+
+    Returns:
+        The adjusted model_directory that should be used in 'test_core_models_in_directory'
+    """
     import rasa.model
 
     p = Path(model_directory)