[Datasets] Add example of using map_batches to filter (#24202)

The documentation says 

> Consider using .map_batches() for better performance (you can implement filter by dropping records).

but there aren't any examples of how to do so.

python/ray/data/dataset.py

@@ -250,6 +250,16 @@ class Dataset(Generic[T]):
             ...     compute=ActorPoolStrategy(2, 8), # doctest: +SKIP
             ...     num_gpus=1) # doctest: +SKIP
 
+            You can use ``map_batches`` to efficiently filter records.
+
+            >>> import ray
+            >>> ds = ray.data.range(10000)  # doctest: +SKIP
+            >>> ds.count()  # doctest: +SKIP
+            10000
+            >>> ds = ds.map_batches(lambda batch: [x for x in batch if x % 2 == 0])  # doctest: +SKIP  # noqa: #501
+            >>> ds.count()  # doctest: +SKIP
+            5000
+
         Time complexity: O(dataset size / parallelism)
 
         Args: