Modify documents of flags in advanced_guide (#2179) (#2189)

test=develop
As the title

doc/fluid/advanced_guide/flags/flags_cn.rst

@@ -2,11 +2,22 @@
 环境变量FLAGS
 ==================
 
+调用说明
+----------
+
+PaddlePaddle中的环境变量FLAGS支持两种设置方式。
+
+- 通过export来设置环境变量，如 :code:`export FLAGS_eager_delete_tensor_gb = 1.0` 。
+
+- 通过API：:code:`get_flag` 和 :code:`set_flags` 来打印和设置环境变量FLAGS。API使用详情请参考 :ref:`cn_api_fluid_get_flags` 与 :ref:`cn_api_fluid_set_flags` 。
+
+
+环境变量FLAGS功能分类
+----------------------
 
 ..  toctree::
     :maxdepth: 1
 
-
     cudnn_cn.rst
     data_cn.rst
     debug_cn.rst

doc/fluid/advanced_guide/flags/flags_en.rst

@@ -2,6 +2,17 @@
 FLAGS
 ==================
 
+Usage
+------
+These FLAGS in PaddlePaddle can be set in two ways.
+
+- Set the FLAGS through export. For example: :code:`export FLAGS_eager_delete_tensor_gb = 1.0` .
+
+- Through :code:`get_flags` and :code:`set_flags` to print and set the environment variables. For more information of using these API, please refer to :ref:`api_fluid_get_flags` and :ref:`api_fluid_get_flags` .
+
+
+FLAGS Quick Search
+------------------
 
 ..  toctree::
     :maxdepth: 1