docstring update

Author: davidsbatista

integrations/opensearch/src/haystack_integrations/document_stores/opensearch/document_store.py

@@ -1201,6 +1201,8 @@ async def count_documents_by_filter_async(self, filters: dict) -> int:
     def _build_cardinality_aggregations(index_mapping: dict[str, Any]) -> dict[str, Any]:
         """
         Builds cardinality aggregations for all metadata fields in the index mapping.
+
+        See: https://docs.opensearch.org/latest/aggregations/metric/cardinality/
         """
         aggs = {}
         for field_name in index_mapping.keys():