message: Update Nesterov implementation docs
  Clarification that this is a modified version of the algorithm which is only correct under certain conditions
  Fixes
diff: @@ -59,6 +59,10 @@ class MomentumOptimizer(optimizer.Optimizer):
  This implementation always computes gradients at the value of the
  variable(s) passed to the optimizer. Using Nesterov Momentum makes the
  variable(s) track the values called `theta_t + mu*v_t` in the paper.
  + This implementation is an approximation of the original formula, valid
  + for high values of momentum. It will compute the "adjusted gradient" in NAG
  + by assuming that the new gradient will be estimated by the current
  + average gradient plus the product of momentum and the change in the average gradient.
  @compatibility(eager)
  When eager execution is enabled, `learning_rate` and `momentum` can each be
  