add a hint for BCR

Author: sfc-gh-yuwang

src/snowflake/snowpark/dataframe_reader.py

@@ -1071,6 +1071,7 @@ def xml(self, path: str, _emit_ast: bool = True) -> DataFrame:
             df._ast_id = stmt.uid
 
         # cast to input custom schema type
+        # TODO: SNOW-2923003: remove single quote after server side BCR is done
         if self._user_schema:
             cols = [
                 df[single_quote(field._name)]