Clarify null-HWND doc note in ForegroundWindow

internal/winapi/winapi.go

@@ -17,7 +17,8 @@ var (
 // ForegroundWindow returns the current foreground window's handle (as a uintptr
 // so platform-neutral callers need not import windows), its title, and its
 // on-task class (process exe base name; see ClassFromImagePath). ok is false
-// when there is no foreground window (e.g. secure desktop / lock screen).
+// when GetForegroundWindow returns null, which happens transiently during focus
+// changes and some secure-desktop transitions (UAC / lock screen).
 func ForegroundWindow() (hwnd uintptr, title, class string, ok bool) {
 	h := windows.GetForegroundWindow()
 	if h == 0 {