math/rand: improve NewZipf documentation

Change-Id: I78f0f9ac33db153c4a0d7f9866da20ebcd3d3639
Reviewed-on: https://go-review.googlesource.com/11872Reviewed-by: Rob Pike <r@golang.org>

src/math/rand/zipf.go

@@ -32,8 +32,10 @@ func (z *Zipf) hinv(x float64) float64 {
 	return math.Exp(z.oneminusQinv*math.Log(z.oneminusQ*x)) - z.v
 }
 
-// NewZipf returns a Zipf generating variates p(k) on [0, imax]
-// proportional to (v+k)**(-s) where s>1 and k>=0, and v>=1.
+// NewZipf returns a Zipf variate generator.
+// The generator generates values k ∈ [0, imax]
+// such that P(k) is proportional to (v + k) ** (-s).
+// Requirements: s > 1 and v >= 1.
 func NewZipf(r *Rand, s float64, v float64, imax uint64) *Zipf {
 	z := new(Zipf)
 	if s <= 1.0 || v < 1 {