Docs - Add note about env slugs being immutable

What does this MR do?

Environment slugs are immutable.

If an environment is renamed, users might expect the slug to automatically rename as well.

This MR clarifies what happens here.

Related issues

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/environments.md

@@ -131,15 +131,27 @@ In summary, with the above `.gitlab-ci.yml` we have achieved the following:
   job will deploy our code to a staging server while the deployment
   will be recorded in an environment named `staging`.
 
-> Starting with GitLab 8.15, the environment name is exposed to the Runner in
-> two forms: `$CI_ENVIRONMENT_NAME`, and `$CI_ENVIRONMENT_SLUG`. The first is
-> the name given in `.gitlab-ci.yml` (with any variables expanded), while the
-> second is a "cleaned-up" version of the name, suitable for use in URLs, DNS,
-> etc.
->
-> Starting with GitLab 9.3, the environment URL is exposed to the Runner via
-> `$CI_ENVIRONMENT_URL`. The URL is expanded from `.gitlab-ci.yml`, or if
-> the URL was not defined there, the external URL from the environment is used.
+#### Environment variables and Runner
+
+Starting with GitLab 8.15, the environment name is exposed to the Runner in
+two forms:
+
+- `$CI_ENVIRONMENT_NAME`. The name given in `.gitlab-ci.yml` (with any variables
+  expanded).
+- `$CI_ENVIRONMENT_SLUG`. A "cleaned-up" version of the name, suitable for use in URLs,
+  DNS, etc.
+
+If you change the name of an existing environment, the:
+
+- `$CI_ENVIRONMENT_NAME` variable will be updated with the new environment name.
+- `$CI_ENVIRONMENT_SLUG` variable will remain unchanged to prevent unintended side
+  effects.
+
+Starting with GitLab 9.3, the environment URL is exposed to the Runner via
+`$CI_ENVIRONMENT_URL`. The URL is expanded from either:
+
+- `.gitlab-ci.yml`.
+- The external URL from the environment if not defined in `.gitlab-ci.yml`.
 
 ### Configuring manual deployments