Adding doc for 2.0 acosh

tensorflow/core/api_def/base_api/api_def_Acosh.pbtxt

@@ -1,4 +1,10 @@
 op {
   graph_op_name: "Acosh"
   summary: "Computes inverse hyperbolic cosine of x element-wise."
+  description: <<END
+Given an input tensor, the function computes inverse hyperbolic cosine of every element.
+Input range is [1, infinity). It return `nan` if the input lies outside the range.
+```python
+tensor `x` is [-2, -0.5, 1, 1.2, 200, 10000]
+tf.acosh(x) ==> [nan nan 0. 0.62236255 5.9914584 9.903487]
 }