Add more information for pre and post

What does this MR do?

Adds more content as a follow up to: !18664 (merged).

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set and that you merge the equivalent EE MR before the CE MR if both exist.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.

doc/ci/yaml/README.md

@@ -329,6 +329,29 @@ The following stages are available to every pipeline:
 
 User-defined stages are executed after `.pre` and before `.post`.
 
+The order of `.pre` and `.post` can not be changed, even if defined out of order in `.gitlab-ci.yml`.
+For example, the following are equivalent configuration:
+
+- Configured in order:
+
+  ```yml
+  stages:
+    - .pre
+    - a
+    - b
+    - .post
+  ```
+
+- Configured out of order:
+
+  ```yml
+  stages:
+    - a
+    - .pre
+    - b
+    - .post
+  ```
+
 ### `stage`
 
 `stage` is defined per-job and relies on [`stages`](#stages) which is defined