Elu specification refactoring (#5012)

* Review spec of Elu operation

* Change reference link to abstract

* Address review comments related to wording

docs/ops/activation/Elu_1.md

@@ -8,29 +8,58 @@
 
 **Detailed Description**
 
-For each element from the input tensor calculates corresponding
-element in the output tensor with the following formula:
+*Elu* operation is introduced in this [article](https://arxiv.org/abs/1511.07289v3).
+It performs element-wise activation function on a given input tensor, based on the following mathematical formula:
+
 \f[
-elu(x) = \left\{\begin{array}{ll}
-    alpha(e^{x} - 1) \quad \mbox{if } x < 0 \\
-    x \quad \mbox{if } x \geq  0
+Elu(x) = \left\{\begin{array}{r}
+    x \qquad \mbox{if } x >  0 \\
+    \alpha(e^{x} - 1) \quad \mbox{if } x \leq 0
 \end{array}\right.
 \f]
 
+where α corresponds to *alpha* attribute.
+
+*Elu* is equivalent to *ReLU* operation when *alpha* is equal to zero. 
+
 **Attributes**
 
 * *alpha*
 
   * **Description**: scale for the negative factor
-  * **Range of values**: arbitrary floating point number
-  * **Type**: float
+  * **Range of values**: non-negative arbitrary floating-point number
+  * **Type**: `float`
   * **Default value**: none
   * **Required**: *yes*
 
 **Inputs**:
 
-*   **1**: Input tensor x of any floating point type. Required.
+*   **1**: A tensor of type `T` and arbitrary shape. **Required**.
 
 **Outputs**:
 
-*   **1**: Result of Elu function applied to the input tensor *x*. Floating point tensor with shape and type matching the input tensor.
+*   **1**: The result of element-wise *Elu* function applied to the input tensor. A tensor of type `T` and the same shape as input tensor.
+
+**Types**
+
+* *T*: arbitrary supported floating-point type.
+
+**Example**
+
+```xml
+<layer ... type="Elu">
+    <data alpha="1.0"/>
+    <input>
+        <port id="0">
+            <dim>1</dim>
+            <dim>128</dim>
+        </port>
+    </input>
+    <output>
+        <port id="1">
+            <dim>1</dim>
+            <dim>128</dim>
+        </port>
+    </output>
+</layer>
+```