message: Improve Adadelta doc.
diff: @@ -41,13 +41,14 @@ class Adadelta(optimizer_v2.OptimizerV2):
  Initialization:
  - $$accum_g_0 := 0 \text{(Initialize gradient 2nd order moment vector)}$$
  - $$accum_x_0 := 0 \text{(Initialize variable update 2nd order moment vector)}$$
  + $$E[g^2]_0 := 0 \text{(Initialize gradient 2nd order moment vector)}$$
  + $$E[\Delta x^2]_0 := 0 \text{(Initialize 2nd order variable update)}$$
  $$t := t + 1$$
  - $$accum_g_t := rho * accum_g_{t-1} + (1 - rho) * g * g$$
  - $$delta = -\sqrt{accum_x_{t-1}} / (\sqrt{accum_g_{t-1}} + \epsilon)$$
  - $$accum_x_t := rho * accum_x_{t-1} + (1 - rho) * delta * delta$$
  + $$E[g^2]_t := \rho * E[g^2]_{t-1} + (1 - \rho) * g^2$$
  + $$\Delta x_t = -RMS[\Delta x]_{t-1} * g_t / RMS[g]_t$$
  + $$E[\Delta x^2]_t := \rho * E[\Delta x^2]_{t-1} + (1 - \rho) * \Delta x_t^2$$
  + $$x_t := x_{t-1} + \Delta x_{t}
  References
  See [M. D. Zeiler](http://arxiv.org/abs/1212.5701)
  