math/rand: add concurrency warning to overview comment

Change-Id: I52efa7aa72a23256e5ca56470ffeba975ed8f739
Reviewed-on: https://go-review.googlesource.com/48760Reviewed-by: Bryan Mills <bcmills@google.com>

src/math/rand/rand.go

@@ -8,7 +8,8 @@
 // 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.
+// The default Source is safe for concurrent use by multiple goroutines, but
+// Sources created by NewSource are not.
 //
 // For random numbers suitable for security-sensitive work, see the crypto/rand
 // package.