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 <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Reviewed-by: Sameer Ajmani <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: kamikazechaser

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