Add YAML keys that user must not override

doc/user/application_security/coverage_fuzzing/index.md

@@ -49,6 +49,14 @@ targets. Each fuzz target **must** have a separate job. For example, the
 [go-fuzzing-example project](https://gitlab.com/gitlab-org/security-products/demos/go-fuzzing-example)
 contains one job that extends `.fuzz_base` for its single fuzz target.
 
+Note that the hidden job `.fuzz_base` uses several YAML keys that you must not override in your own
+job. If you include these keys in your own job, you must copy their original content. These keys
+are:
+
+- `before_script`
+- `artifacts`
+- `rules`
+
 The `my_fuzz_target` job (the separate job for your fuzz target) does the following:
 
 - Extends `.fuzz_base`.