update api document

mindspore/dataset/transforms/c_transforms.py

@@ -31,7 +31,10 @@ class OneHot(cde.OneHotOp):
     Tensor operation to apply one hot encoding.
 
     Args:
-        num_classes (int): Number of classes of the label.
+        num_classes (int): Number of classes of the label, it should be bigger than feature size.
+
+    Raises:
+        RuntimeError: feature size is bigger than num_classes.
     """
 
     @check_num_classes