add comments for nsexec.c

It would be easier for people to understand how nsexec really
works.

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>

nsenter/nsexec.c

@@ -137,6 +137,8 @@ void nsexec()
 	}
 
 	if (setjmp(env) == 1) {
+		// Child
+
 		if (setsid() == -1) {
 			pr_perror("setsid failed");
 			exit(1);
@@ -162,7 +164,11 @@ void nsexec()
 		// Finish executing, let the Go runtime take over.
 		return;
 	}
+	// Parent
 
+	// We must fork to actually enter the PID namespace, use CLONE_PARENT
+	// so the child can have the right parent, and we don't need to forward
+	// the child's exit code or resend its death signal.
 	child = clone_parent(&env);
 	if (child < 0) {
 		pr_perror("Unable to fork");