math/rand: document that NewSource sources race

While it was previously explicitly documented that "the default Source"
is safe for concurrent use, a careless reader can interpret that as
meaning "the implementation of the Source interface created by functions
in this package" rather than "the default shared Source used by
top-level functions". Be explicit that the Source returned by NewSource
is not safe for use by multiple goroutines.

Fixes #3611.

Change-Id: Iae4bc04c3887ad6e2491e36e38feda40324022c5
Reviewed-on: https://go-review.googlesource.com/25501Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/math/rand/rand.go

@@ -24,6 +24,8 @@ type Source interface {
 }
 
 // NewSource returns a new pseudo-random Source seeded with the given value.
+// Unlike the default Source used by top-level functions, this source is not
+// safe for concurrent use by multiple goroutines.
 func NewSource(seed int64) Source {
 	var rng rngSource
 	rng.Seed(seed)