Add note about CI_ENVIRONMENT_SLUG being truncated

The variable contents are limited to just 24 characters and that should
be a part of the documentation.

(cherry picked from commit 8b51eb0e690c0f68010eb0c6f760f05b71abb442)

doc/ci/variables/predefined_variables.md

@@ -50,7 +50,7 @@ There are also [Kubernetes-specific deployment variables](../../user/project/clu
 | `CI_DEPLOY_USER`                         | 10.8   | all    | The authentication username of the [GitLab Deploy Token](../../user/project/deploy_tokens/index.md#gitlab-deploy-token), if the project has one. |
 | `CI_DISPOSABLE_ENVIRONMENT`              | all    | 10.1   | Only available if the job is executed in a disposable environment (something that is created only for this job and disposed of/destroyed after the execution - all executors except `shell` and `ssh`). `true` when available. |
 | `CI_ENVIRONMENT_NAME`                    | 8.15   | all    | The name of the environment for this job. Available if [`environment:name`](../yaml/README.md#environmentname) is set. |
-| `CI_ENVIRONMENT_SLUG`                    | 8.15   | all    | The simplified version of the environment name, suitable for inclusion in DNS, URLs, Kubernetes labels, and so on. Available if [`environment:name`](../yaml/README.md#environmentname) is set. |
+| `CI_ENVIRONMENT_SLUG`                    | 8.15   | all    | The simplified version of the environment name, suitable for inclusion in DNS, URLs, Kubernetes labels, and so on. Available if [`environment:name`](../yaml/README.md#environmentname) is set. The slug is [truncated to 24 characters](https://gitlab.com/gitlab-org/gitlab/-/issues/20941). |
 | `CI_ENVIRONMENT_URL`                     | 9.3    | all    | The URL of the environment for this job. Available if [`environment:url`](../yaml/README.md#environmenturl) is set. |
 | `CI_HAS_OPEN_REQUIREMENTS`               | 13.1   | all    | Only available if the pipeline's project has an open [requirement](../../user/project/requirements/index.md). `true` when available. |
 | `CI_JOB_ID`                              | 9.0    | all    | The internal ID of the job, unique across all jobs in the GitLab instance. |