Enrich English declarative Doc (PaddlePaddle#26538)

As the title.

python/paddle/fluid/dygraph/jit.py

@@ -132,13 +132,16 @@ def _declarative_(dygraph_func):
     """
     Converts imperative dygraph APIs into declarative function APIs. Decorator
     @declarative handles the Program and Executor of static mode and returns
-    the result as a dygraph VarBase.
+    the result as dygraph Tensor(s). Users could use the returned dygraph
+    Tensor(s) to do imperative training, inference, or other operations. If the
+    decorated function calls other imperative function, the called one will be
+    converted into declarative function as well.
 
     Args:
         dygraph_func (callable): callable imperative function.
 
     Returns:
-        VarBase: containing the numerical result.
+        Tensor(s): containing the numerical result.
 
     Examples:
         .. code-block:: python
@@ -147,6 +150,7 @@ def _declarative_(dygraph_func):
           import numpy as np
           from paddle.fluid.dygraph.jit import declarative
 
+          fluid.enable_dygraph()
 
           @declarative
           def func(x):