Add docs for `INTERNAL_TOKEN_URI` (#7234)

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

@@ -195,6 +195,8 @@ Values containing `#` or `;` must be quoted using `` ` `` or `"""`.
 - `DISABLE_GIT_HOOKS`: **false**: Set to `true` to prevent all users (including admin) from creating custom
    git hooks.
 - `IMPORT_LOCAL_PATHS`: **false**: Set to `false` to prevent all users (including admin) from importing local path on server.
+- `INTERNAL_TOKEN`: **\<random at every install if no uri set\>**: Secret used to validate communication within Gitea binary.
+- `INTERNAL_TOKEN_URI`: **<empty>**: Instead of defining internal token in the configuration, this configuration option can be used to give Gitea a path to a file that contains the internal token (example value: `file:/etc/gitea/internal_token`)
 
 ## OpenID (`openid`)