runtime: note a minor issue with GODEUG=gcstoptheworld

Change-Id: I91cda8d88b0852cd0f868d33c594206bcca0c386
Reviewed-on: https://go-review.googlesource.com/16352Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgc.go

@@ -916,6 +916,9 @@ func startGC(mode gcMode, forceTrigger bool) {
 	releasem(mp)
 	mp = nil
 
+	// TODO: In gcstoptheworld debug mode, multiple goroutines may
+	// detect the heap trigger simultaneously and then start
+	// multiple STW GCs, which will run sequentially.
 	if debug.gcstoptheworld == 1 {
 		mode = gcForceMode
 	} else if debug.gcstoptheworld == 2 {