Gelu specification refactoring (#5001)

* Review spec of Gelu operation

* Address review comments related to wording

docs/ops/activation/GELU_2.md

@@ -2,35 +2,40 @@
 
 **Versioned name**: *Gelu-2*
 
-**Category**: *Activation*
+**Category**: *Activation function*
 
-**Short description**: [Reference](https://pytorch.org/docs/stable/nn.functional.html#gelu)
+**Short description**: Gaussian error linear unit element-wise activation function.
 
-**Detailed description**: [Reference](https://arxiv.org/abs/1606.08415)
+**Detailed description**
+
+*Gelu* operation is introduced in this [article](https://arxiv.org/abs/1606.08415).
+It performs element-wise activation function on a given input tensor, based on the following mathematical formula:
+
+\f[
+    Gelu(x) = x\cdot\Phi(x) = x\cdot\frac{1}{2}\cdot\left[1 + erf\left(x/\sqrt{2}\right)\right]
+\f]
+
+where Φ(x) is the Cumulative Distribution Function for Gaussian Distribution.
+
+Additionally, *Gelu* function may be approximated as follows:
+
+\f[
+    Gelu(x) \approx 0.5\cdot x\cdot \left(1 + \tanh\left[\sqrt{2/\pi} \cdot (x + 0.044715 \cdot x^3)\right]\right)
+\f]
 
 **Attributes**: *Gelu* operation has no attributes.
 
-**Mathematical Formulation**
-Gelu(x)=x*Φ(x), where Φ(x) is the Cumulative Distribution Function for Gaussian Distribution.
-The following equivalent combination is recognized and fused into single Gelu op: 
-
-\f[
-    Gelu(x) = 0.5*x*(1.0 + erf((x) / \sqrt{2})
-\f]
-
-Similarly, the following Gelu approximation (typical for the TensorFlow*) is recognized and fused into single Gelu op 
-
-\f[
-    Gelu(x) \approx 0.5x(1.0 + tanh(\sqrt{2.0/pi} * (x + 0.044715 * x ^ 3))
-\f]
-
 **Inputs**:
 
-*   **1**: Multidimensional input tensor. Required.
+*   **1**: A tensor of type `T` and arbitrary shape. **Required**.
 
 **Outputs**:
 
-*   **1**: Floating point tensor with shape and type matching the input tensor.
+*   **1**: The result of element-wise *Gelu* 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**