Document refactor

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doc/python/python_api.rst

@@ -1,6 +1,8 @@
 Python API Reference
 ====================
-This page gives the Python API reference of xgboost.
+This page gives the Python API reference of xgboost, please also refer to Python Package Introduction for more information about python package.
+
+The document in this page is automatically generated by sphinx. The content do not render at github, you can view it at http://xgboost.readthedocs.org/en/latest/python/python_api.html
 
 Core Data Structure
 -------------------
@@ -33,4 +35,3 @@ Scikit-Learn API
 .. autoclass:: xgboost.XGBClassifier
     :members:
     :show-inheritance:
-