math/rand: fix comment about bits of seed used by the default Source

Fixes #15788

Change-Id: I5a1fd1e5992f1c16cf8d8437d742bf02e1653b9c
Reviewed-on: https://go-review.googlesource.com/23461
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Author: kostya-sh

src/math/rand/rand.go

@@ -179,8 +179,8 @@ var globalRand = New(&lockedSource{src: NewSource(1)})
 
 // Seed uses the provided seed value to initialize the default Source to a
 // deterministic state. If Seed is not called, the generator behaves as
-// if seeded by Seed(1). Only uses the bottom 31 bits of seed; the top 33
-// bits are ignored.
+// if seeded by Seed(1). Seed values that have the same remainder when
+// divided by 2^31-1 generate the same pseudo-random sequence.
 func Seed(seed int64) { globalRand.Seed(seed) }
 
 // Int63 returns a non-negative pseudo-random 63-bit integer as an int64