message: 1. Rename n to a to match actual API, see
  2. Add a comment on which values of `a` are valid.  (Negative or fractional values for `a` as tf.float lead to NaN.)
diff: @@ -5,8 +5,9 @@ op {
  The polygamma function is defined as:
  -\\(\psi^{(n)}(x) = \frac{d^n}{dx^n} \psi(x)\\)
  +\\(\psi^{(a)}(x) = \frac{d^a}{dx^a} \psi(x)\\)
  where \\(\psi(x)\\) is the digamma function.
  +The polygamma function is defined only for non-negative integer orders \\a\\.
  END
  }
  