Merge branch 'update-pre-post-stages-documentation' into 'master'

Update documentation for pre/post stages

See merge request gitlab-org/gitlab!27410

doc/ci/yaml/README.md

@@ -460,6 +460,9 @@ For example, the following are equivalent configuration:
     - b
   ```
 
+NOTE: **Note:**
+A pipeline will not be created if it only contains jobs in `.pre` or `.post` stages.
+
 ### `stage`
 
 `stage` is defined per-job and relies on [`stages`](#stages) which is defined