[doc] Add example for buffered and cache, test=develop, test=document_fix (#2677)

* Add example for buffered and cache, test=develop, test=document_fix

* up, test=develop;test=document_fix

doc/paddle/api/paddle/io/buffered_cn.rst

-.. _cn_api_fluid_io_buffered:
+.. _cn_api_io_buffered:
 
 buffered
 -------------------------------
 
-.. py:function:: paddle.fluid.io.buffered(reader, size)
+.. py:function:: paddle.io.buffered(reader, size)
 
 
 
@@ -15,3 +15,20 @@ buffered
     - **size** (int) – 最大buffer的大小
 
 返回:缓存的reader（读取器）
+
+**代码示例**
+
+..  code-block:: python
+
+    import paddle
+
+    def reader():
+        for i in range(3):
+            yield i
+
+    # Create a buffered reader, and the buffer size is 2.
+    buffered_reader = paddle.io.buffered(reader, 2)
+
+    # Output: 0 1 2
+    for i in buffered_reader():
+        print(i)

doc/paddle/api/paddle/io/cache_cn.rst

-.. _cn_api_fluid_io_cache:
+.. _cn_api_io_cache:
 
 cache
 -------------------------------
 
-.. py:function:: paddle.fluid.io.cache(reader)
+.. py:function:: paddle.io.cache(reader)
 
 
 
@@ -15,4 +15,21 @@ cache
 
 返回：每次都会从内存中yields数据的一个装饰reader。
 
-返回类型：生成器
\ No newline at end of file
+返回类型：数据保存在内存的reader（读取器）
+
+**代码示例**
+
+..  code-block:: python
+
+    import paddle
+
+    def reader():
+        for i in range(3):
+            yield i
+
+    # All data is cached into memory
+    cached_reader = paddle.io.cache(reader)
+
+    # Output: 0 1 2
+    for i in cached_reader():
+        print(i)