Update index_en.rst (#9286)

* Update index_en.rst

Update en version

* Update index_en.rst

Update refer to commits and thank you for the suggestion.

doc/v2/howto/capi/index_en.rst

@@ -1,6 +1,23 @@
-C-API Prediction Library
+C-API Inference Library
 ========================
 
+After we train a neural network, we use it to do inference. Inference is the process of preparing input data and propagating it through the model to produce the result.
+
+Compared with model training, prediction has the following features:
+
+#. Inference does not require backpropagation and parameter updates, as required during training.
+#. Labels are not needed in prediction.
+#. Most of the time, predictions need to be integrated with the user system.
+
+Therefore, the model prediction SDK needs to be designed separately and has the following features:
+
+#. The predictive SDK does not include backpropagation and parameter updates to reduce the size of the SDK.
+#. The predictive SDK needs a simple user interface for ease of use.
+#. Since the input data may have a variety of structures, the format of the input data is clearly and compactly packaged.
+#. In order to be compatible with user's system, the SDK's interface must conform to the C-standard interface.
+
+PaddlePaddle provides C-API to solve the above problem. Following are the guidelines to use the C-API:
+
 ..  toctree::
   :maxdepth: 1