rate: add documentation on Limiter concurrent usage

Closes golang/go#17261

Change-Id: Ifeacc8df45fb59e7781ddcc35c2fde0aa80188eb
Reviewed-on: https://go-review.googlesource.com/c/time/+/530855
Auto-Submit: Sameer Ajmani <sameer@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Sameer Ajmani <sameer@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

rate/rate.go

@@ -52,6 +52,8 @@ func Every(interval time.Duration) Limit {
 // or its associated context.Context is canceled.
 //
 // The methods AllowN, ReserveN, and WaitN consume n tokens.
+//
+// Limiter is safe for simultaneous use by multiple goroutines.
 type Limiter struct {
 	mu     sync.Mutex
 	limit  Limit