math/rand: mention that the default Source is thread-safe

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/11709043

src/pkg/math/rand/rand.go

@@ -8,6 +8,7 @@
 // Float64 and Int, use a default shared Source that produces a deterministic
 // sequence of values each time a program is run. Use the Seed function to
 // initialize the default Source if different behavior is required for each run.
+// The default Source is safe for concurrent use by multiple goroutines.
 package rand
 
 import "sync"