chore(github-enterprise): add note about resolvable installation URL (#12410)

Author: cathteng

docs/organization/integrations/source-code-mgmt/github/index.mdx

@@ -159,7 +159,7 @@ If you're using GitHub Enterprise Cloud, follow the [instructions for GitHub](/o
 
    ![Add Installation](./img/github-e-add-installation.png)
 
-4. Fill out the following form with information from your GitHub apps configuration page.
+4. Fill out the following form with information from your GitHub apps configuration page. The installation URL must be resolvable over the Internet.
 
    ![GitHub Enterprise configuration form](./img/github-e-form.png)