sync: make clear that WaitGroup.Done decrements by one

Change-Id: Ief076151739147378f8ca35cd09aabb59c3c9a52
Reviewed-on: https://go-review.googlesource.com/46350
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/sync/waitgroup.go

@@ -95,7 +95,7 @@ func (wg *WaitGroup) Add(delta int) {
 	}
 }
 
-// Done decrements the WaitGroup counter.
+// Done decrements the WaitGroup counter by one.
 func (wg *WaitGroup) Done() {
 	wg.Add(-1)
 }