Update GitHUb App sync error scenario in docs for GitHub enterprise c…

docs/admin/code_hosts/github.mdx

@@ -160,6 +160,16 @@ To use a GitHub Enterprise App with Sourcegraph:
 1. Create the Enterprise App in your GitHub Enterprise settings (see [GitHub's documentation](https://docs.github.com/en/enterprise-cloud@latest/admin/managing-your-enterprise-account/creating-github-apps-for-your-enterprise))
 2. Use the [**Add an existing GitHub App**](#adding-an-existing-github-app) option in Sourcegraph to connect it
 
+If you get an error message similar to following error while adding a GitHub App connection :
+
+```
+"request to https://api.github.com/app/installations?page=1 returned status 403: Although you appear to have the correct authorization credentials, the `testsourcegraph` organization has an IP allow list enabled, and your IP address is not permitted to access this resource."
+
+```
+
+ It means that the IP allowlist is enabled at the organisation level and you will need to add sourcegraph's IP addresseses to this allowlist in order for the app connection to work (see [GitHub's documentation](https://docs.github.com/en/enterprise-cloud@latest/organizations/keeping-your-organization-secure/managing-security-settings-for-your-organization/managing-allowed-ip-addresses-for-your-organization#adding-an-allowed-ip-address))
+
+
 ### Uninstalling an App
 
 You can uninstall a GitHub App from a namespace or remove it altogether at any time.