Give reason why Slack app creation is disabled

This change gives the reason why the GitLab for Slack app cannot be
created through the API, as the topic is coming up in
https://gitlab.com/gitlab-org/terraform-provider-gitlab/-/issues/1469#note_1837736990
and should be clarified for readers.

doc/api/integrations.md

@@ -806,7 +806,10 @@ GET /projects/:id/integrations/github
 
 Update the GitLab for Slack app integration for a project.
 
-You cannot create a GitLab for Slack app. You must [install the app](../user/project/integrations/gitlab_slack_application.md#install-the-gitlab-for-slack-app) from the GitLab UI.
+You cannot create a GitLab for Slack app through the API because the integration
+requires an OAuth 2.0 token that you cannot get from the GitLab API alone.
+Instead, you must [install the app](../user/project/integrations/gitlab_slack_application.md#install-the-gitlab-for-slack-app) from the GitLab UI.
+You can then use this API endpoint to update the integration.
 
 ```plaintext
 PUT /projects/:id/integrations/gitlab-slack-application