testing: document that ResetTimer also zeros the allocation counters.

Fixes #6998.

R=golang-codereviews, gobot, r
CC=golang-codereviews
https://golang.org/cl/44880044

src/pkg/testing/benchmark.go

@@ -74,7 +74,7 @@ func (b *B) StopTimer() {
 	}
 }
 
-// ResetTimer sets the elapsed benchmark time to zero.
+// ResetTimer zeros the elapsed benchmark time and memory allocation counters.
 // It does not affect whether the timer is running.
 func (b *B) ResetTimer() {
 	if b.timerOn {