runtime: note about improved _Gscan bit handling

Change-Id: I8de5aa64a24e77e0ef876918fcace7668769ebc2
Reviewed-on: https://go-review.googlesource.com/c/go/+/171022Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/runtime2.go

@@ -22,6 +22,13 @@ const (
 	// If you add to this list, add to the list
 	// of "okay during garbage collection" status
 	// in mgcmark.go too.
+	//
+	// TODO(austin): The _Gscan bit could be much lighter-weight.
+	// For example, we could choose not to run _Gscanrunnable
+	// goroutines found in the run queue, rather than CAS-looping
+	// until they become _Grunnable. And transitions like
+	// _Gscanwaiting -> _Gscanrunnable are actually okay because
+	// they don't affect stack ownership.
 
 	// _Gidle means this goroutine was just allocated and has not
 	// yet been initialized.