internal/poll: fix error in increfAndClose documentation

Change-Id: I0c387b6286bc18fd00c6ac4e42bdf175cf89ccb1
Reviewed-on: https://go-review.googlesource.com/83155Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/internal/poll/fd_mutex.go

@@ -65,7 +65,7 @@ func (mu *fdMutex) incref() bool {
 }
 
 // increfAndClose sets the state of mu to closed.
-// It reports whether there is no remaining reference.
+// It returns false if the file was already closed.
 func (mu *fdMutex) increfAndClose() bool {
 	for {
 		old := atomic.LoadUint64(&mu.state)