Refine API doc string

[panyx0718]

It seems all our activation ops suffer from the issue

[panyx0718]

I'm wondering if there is a way to see it's effect on webpage?

I manually print it:

sigmoid

Sigmoid Activation Operator

$$out = \frac{1}{1 + e^{-x}}$$


Args:
    x: Input of Sigmoid operator
       Duplicable: False  Optional: False
    use_mkldnn (BOOLEAN): (bool, default false) Only used in mkldnn kernel
    op_role (INT): The role of this operator
    op_role_var (STRINGS): Optimized for variable

Returns:
    Output of Sigmoid operator

[dzhwinter]

Compile the doc site, then we can preview the effects.