document the section name for "Overall" section (#118)

In certain contexts, the Gitea configuration is put together
by concatenating multiple files. In such a situation, it
becomes hard to override the values in the Overall section
as they get pushed into whatever section is defined last,
unless you know that it can be specified via "DEFAULT".

Signed-off-by: Ashish Kulkarni <[email protected]>

Author: ashkulz

content/doc/advanced/config-cheat-sheet.en-us.md

@@ -23,7 +23,7 @@ All default settings can be found in [app.ini](https://github.com/go-gitea/gitea
 
 Any configuration option that is marked by :exclamation: means that you should keep the default value unless you fully understand what you are doing.
 
-## Overall
+## Overall (`DEFAULT`)
 
 - `APP_NAME`: Application name, change to whatever you want.
 - `RUN_USER`: The user to run Gitea as, we recommend it be `git`; however, change this to whatever your username is if you run Gitea on your personal computer. Gitea may crash if this value is not set properly.

content/doc/advanced/config-cheat-sheet.zh-cn.md

@@ -17,7 +17,7 @@ menu:
 
 这是针对Gitea配置文件的说明，你可以了解Gitea的强大配置。需要说明的是，你的所有改变请修改 `custom/conf/app.ini` 文件而不是源文件。所有默认值可以通过 [app.ini](https://github.com/go-gitea/gitea/blob/master/conf/app.ini) 查看到。如果你发现 `%(X)s` 这样的内容，请查看 [ini](https://github.com/go-ini/ini/#recursive-values) 这里的说明。标注了 :exclamation: 的配置项表明除非你真的理解这个配置项的意义，否则最好使用默认值。
 
-## Overall
+## Overall (`DEFAULT`)
 
 - `APP_NAME`: 应用名称，改成你希望的名字。
 - `RUN_USER`: 运行Gitea的用户，推荐使用 `git`；如果在你自己的个人电脑使用改成你自己的用户名。如果设置不正确，Gitea可能崩溃。