Update IKEv2 docs

- Change 'mobike' from 'yes' to 'no' by default, because it is not
  available on Ubuntu and can prevent the IKEv2 config from loading

docs/ikev2-howto-zh.md

@@ -73,10 +73,12 @@ Libreswan 支持通过使用 RSA 签名算法的 X.509 Machine Certificates 来
    $ cat >> /etc/ipsec.conf <<EOF
      modecfgdns="8.8.8.8, 8.8.4.4"
      encapsulation=yes
-     mobike=yes
+     mobike=no
    EOF
    ```
 
+   **注：** 如果你的服务器运行 Debian 或者 CentOS，并且你想要启用 MOBIKE 支持，可以将上面命令中的 `mobike=no` 换成 `mobike=yes`。**不要**在 Ubuntu 系统上启用该选项。
+
    如果是 Libreswan 3.19-3.22：
 
    ```bash

docs/ikev2-howto.md

@@ -73,10 +73,12 @@ Before continuing, make sure you have successfully <a href="https://github.com/h
    $ cat >> /etc/ipsec.conf <<EOF
      modecfgdns="8.8.8.8, 8.8.4.4"
      encapsulation=yes
-     mobike=yes
+     mobike=no
    EOF
    ```
 
+   **Note:** If your server runs Debian or CentOS and you wish to enable MOBIKE support, replace `mobike=no` with `mobike=yes` in the command above. DO NOT enable this option on Ubuntu systems.
+
    For Libreswan 3.19-3.22:
 
    ```bash