rate: fix documentation of Limiter.ReserveN()

Update documentation string to mention that the returned
Reservation's OK() method returns false if n exceeds the
Limiter's burst size.

Fixes golang/go#38434

Change-Id: Ic549100189caad2ae6cda8379bfd8fa04b02c415
Reviewed-on: https://go-review.googlesource.com/c/time/+/228357
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

rate/rate.go

@@ -196,7 +196,7 @@ func (lim *Limiter) Reserve() *Reservation {
 
 // ReserveN returns a Reservation that indicates how long the caller must wait before n events happen.
 // The Limiter takes this Reservation into account when allowing future events.
-// ReserveN returns false if n exceeds the Limiter's burst size.
+// The returned Reservation’s OK() method returns false if n exceeds the Limiter's burst size.
 // Usage example:
 //   r := lim.ReserveN(time.Now(), 1)
 //   if !r.OK() {