docs: api documentation for agents/eval/scoring/datasets (#1400)

# What does this PR do?

- add some docs to OpenAPI for agents/eval/scoring/datasetio

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan
- read

[//]: # (## Documentation)

llama_stack/apis/datasetio/datasetio.py

@@ -14,6 +14,14 @@ from llama_stack.schema_utils import json_schema_type, webmethod
 
 @json_schema_type
 class PaginatedRowsResult(BaseModel):
+    """
+    A paginated list of rows from a dataset.
+
+    :param rows: The rows in the current page.
+    :param total_count: The total number of rows in the dataset.
+    :param next_page_token: The token to get the next page of rows.
+    """
+
     # the rows obey the DatasetSchema for the given dataset
     rows: List[Dict[str, Any]]
     total_count: int
@@ -36,7 +44,15 @@ class DatasetIO(Protocol):
         rows_in_page: int,
         page_token: Optional[str] = None,
         filter_condition: Optional[str] = None,
-    ) -> PaginatedRowsResult: ...
+    ) -> PaginatedRowsResult:
+        """Get a paginated list of rows from a dataset.
+
+        :param dataset_id: The ID of the dataset to get the rows from.
+        :param rows_in_page: The number of rows to get per page.
+        :param page_token: The token to get the next page of rows.
+        :param filter_condition: (Optional) A condition to filter the rows by.
+        """
+        ...
 
     @webmethod(route="/datasetio/rows", method="POST")
     async def append_rows(self, dataset_id: str, rows: List[Dict[str, Any]]) -> None: ...