math/rand: Doc fix for how many bits Seed uses

Document the fact that the default Source uses only
the bottom 31 bits of the given seed.

Fixes #15788

Change-Id: If20d1ec44a55c793a4a0a388f84b9392c2102bd1
Reviewed-on: https://go-review.googlesource.com/23352Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/math/rand/rand.go

@@ -179,7 +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).
+// if seeded by Seed(1). Only uses the bottom 31 bits of seed; the top 33
+// bits are ignored.
 func Seed(seed int64) { globalRand.Seed(seed) }
 
 // Int63 returns a non-negative pseudo-random 63-bit integer as an int64