Fix actions design about default actions download url (go-gitea#30360)

Fix go-gitea#30359

docs/content/usage/actions/design.en-us.md

@@ -104,7 +104,7 @@ However, if a job container tries to fetch code from localhost, it will fail bec
 ### Connection 3, act runner to internet
 
 When you use some actions like `actions/checkout@v4`, the act runner downloads the scripts, not the job containers.
-By default, it downloads from [gitea.com](http://gitea.com/), so it requires access to the internet.
+By default, it downloads from [github.com](http://github.com/), so it requires access to the internet. If you configure the `DEFAULT_ACTIONS_URL` to `self`, then it will download from your Gitea instance by default. Then it will not connect to internet when downloading the action itself.
 It also downloads some docker images from Docker Hub by default, which also requires internet access.
 
 However, internet access is not strictly necessary.

docs/content/usage/actions/design.zh-cn.md

@@ -105,7 +105,8 @@ act runner 必须能够连接到Gitea以接收任务并发送执行结果回来
 ### 连接 3，act runner到互联网
 
 当您使用诸如 `actions/checkout@v4` 的一些Actions时，act runner下载的是脚本，而不是Job容器。
-默认情况下，它从[gitea.com](http://gitea.com/)下载，因此需要访问互联网。
+默认情况下，它从[github.com](http://github.com/)下载，因此需要访问互联网。如果您设置的是 self，
+那么默认将从您的当前Gitea实例下载，那么此步骤不需要连接到互联网。
 它还默认从Docker Hub下载一些Docker镜像，这也需要互联网访问。
 
 然而，互联网访问并不是绝对必需的。