Merge branch 'docs-mr-variables' into 'master'

Clarify MR target and source SHA variables

See merge request gitlab-org/gitlab!55585

doc/ci/variables/predefined_variables.md

@@ -135,12 +135,12 @@ These variables are available when:
 | `CI_MERGE_REQUEST_PROJECT_URL`         | 11.6   | all    | The URL of the project of the merge request. For example, `http://192.168.10.15:3000/namespace/awesome-project`. |
 | `CI_MERGE_REQUEST_REF_PATH`            | 11.6   | all    | The ref path of the merge request. For example, `refs/merge-requests/1/head`. |
 | `CI_MERGE_REQUEST_SOURCE_BRANCH_NAME`  | 11.6   | all    | The source branch name of the merge request. |
-| `CI_MERGE_REQUEST_SOURCE_BRANCH_SHA`   | 11.9   | all    | The HEAD SHA of the source branch of the merge request. Only available in [merged results pipelines](../merge_request_pipelines/pipelines_for_merged_results/index.md). **(PREMIUM)** |
+| `CI_MERGE_REQUEST_SOURCE_BRANCH_SHA`   | 11.9   | all    | The HEAD SHA of the source branch of the merge request. The variable is empty in merge request pipelines. The SHA is present only in [merged results pipelines](../merge_request_pipelines/pipelines_for_merged_results/index.md). **(PREMIUM)** |
 | `CI_MERGE_REQUEST_SOURCE_PROJECT_ID`   | 11.6   | all    | The ID of the source project of the merge request. |
 | `CI_MERGE_REQUEST_SOURCE_PROJECT_PATH` | 11.6   | all    | The path of the source project of the merge request. |
 | `CI_MERGE_REQUEST_SOURCE_PROJECT_URL`  | 11.6   | all    | The URL of the source project of the merge request. |
 | `CI_MERGE_REQUEST_TARGET_BRANCH_NAME`  | 11.6   | all    | The target branch name of the merge request. |
-| `CI_MERGE_REQUEST_TARGET_BRANCH_SHA`   | 11.9   | all    | The HEAD SHA of the target branch of the merge request. Only available in [merged results pipelines](../merge_request_pipelines/pipelines_for_merged_results/index.md). **(PREMIUM)** |
+| `CI_MERGE_REQUEST_TARGET_BRANCH_SHA`   | 11.9   | all    | The HEAD SHA of the target branch of the merge request. The variable is empty in merge request pipelines. The SHA is present only in [merged results pipelines](../merge_request_pipelines/pipelines_for_merged_results/index.md). **(PREMIUM)** |
 | `CI_MERGE_REQUEST_TITLE`               | 11.9   | all    | The title of the merge request. |
 | `CI_MERGE_REQUEST_EVENT_TYPE`          | 12.3   | all    | The event type of the merge request. Can be `detached`, `merged_result` or `merge_train`. |
 | `CI_MERGE_REQUEST_DIFF_ID`             | 13.7   | all    | The version of the merge request diff. |