fix chain doc, test=develop, test=document_preview (#22070)

* fix chain doc, test=develop, test=document_preview

python/paddle/reader/decorator.py

@@ -151,7 +151,8 @@ def shuffle(reader, buf_size):
 def chain(*readers):
     """
     Use the input data readers to create a chained data reader. The new created reader
-    chains the outputs of input readers together as its output.
+    chains the outputs of input readers together as its output, and it do not change
+    the format of the outputs.
 
     **Note**:
         ``paddle.reader.chain`` is the alias of ``paddle.fluid.io.chain``, and
@@ -162,7 +163,7 @@ def chain(*readers):
     [10, 10, 10],
     [20, 20, 20].
     The chained reader will output:
-    [[0, 0, 0], [10, 10, 10], [20, 20, 20]].
+    [0, 0, 0], [10, 10, 10], [20, 20, 20].
 
     Args:
         readers(list): input data readers.