Explain new sast / dependency_scanning error that was introduced in 13.4

doc/user/application_security/dependency_scanning/index.md

@@ -528,3 +528,7 @@ scanning job might be triggered even if the scanner doesn't support your project
 
 [Python 2 was removed](https://www.python.org/doc/sunset-python-2/) from the `retire.js` analyzer in GitLab 13.7 (analyzer version 2.10.1). Projects using packages
 with a dependency on this version of Python should use `retire.js` version 2.10.0 or lower (for example, `registry.gitlab.com/gitlab-org/security-products/analyzers/retire.js:2.10.0`).
+
+### Error: `dependency_scanning is used for configuration only, and its script should not be executed`
+
+For information on this, see the [GitLab Secure troubleshooting section](../index.md#error-job-is-used-for-configuration-only-and-its-script-should-not-be-executed).

doc/user/application_security/index.md

@@ -691,3 +691,17 @@ Feature.disable(:core_security_mr_widget)
 # For a single project
 Feature.disable(:core_security_mr_widget, Project.find(<project id>))
 ```
+
+### Error: job `is used for configuration only, and its script should not be executed`
+
+[Changes made in GitLab 13.4](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/41260)
+to the `Security/Dependency-Scanning.gitlab-ci.yml` and `Security/SAST.gitlab-ci.yml`
+templates mean that if you enable the `sast` or `dependency_scanning` jobs by setting the `rules` attribute,
+they will fail with the error `(job) is used for configuration only, and its script should not be executed`.
+
+The `sast` or `dependency_scanning` stanzas can be used to make changes to all SAST or Dependency Scanning,
+such as changing `variables` or the `stage`, but they cannot be used to define shared `rules`.
+
+There [is an issue open to improve extendability](https://gitlab.com/gitlab-org/gitlab/-/issues/218444).
+Please upvote the issue to help with prioritization, and
+[contributions are welcomed](https://about.gitlab.com/community/contribute/).

doc/user/application_security/sast/index.md

@@ -720,6 +720,10 @@ affected. Read more in
 
 For information on this, see the [general Application Security troubleshooting section](../../../ci/pipelines/job_artifacts.md#error-message-no-files-to-upload).
 
+### Error: `sast is used for configuration only, and its script should not be executed`
+
+For information on this, see the [GitLab Secure troubleshooting section](../index.md#error-job-is-used-for-configuration-only-and-its-script-should-not-be-executed).
+
 ### Limitation when using rules:exists
 
 The [SAST CI template](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Security/SAST.gitlab-ci.yml)