Add code comment related to setTimeout().unref (#401)

lib/kill.js

@@ -21,6 +21,9 @@ const setKillTimeout = (kill, signal, options, killResult) => {
 		kill('SIGKILL');
 	}, timeout);
 
+	// Guarded because there's no `.unref()` when `execa` is used in the renderer
+	// process in Electron. This cannot be tested since we don't run tests in
+	// Electron.
 	// istanbul ignore else
 	if (t.unref) {
 		t.unref();