Update reference-yaml-mltable.md

Author: SturgeonMi

articles/machine-learning/reference-yaml-mltable.md

@@ -232,6 +232,9 @@ transformations:
       # alternative:
       # version_as_of: 1   
 ```
+> [!IMPORTANT]
+> **Limitation**: `mltable` doesn't support extracting partition keys when reading data from Delta Lake.
+> The `mltable` transformation `extract_columns_from_partition_format` won't work when you are reading Delta Lake data via `mltable`.
 
 ### JSON
 ```yaml
@@ -248,4 +251,4 @@ transformations:
 ## Next steps
 
 - [Install and use the CLI (v2)](how-to-configure-cli.md)
-- [Working with tables in Azure Machine Learning](how-to-mltable.md)
+- [Working with tables in Azure Machine Learning](how-to-mltable.md)