Merge branch 'docs-blunceford-before-sha-fix' into 'master'

Updated explanation of CI_COMMIT_BEFORE_SHA variable

See merge request gitlab-org/gitlab-ce!29913

doc/ci/variables/predefined_variables.md

@@ -30,7 +30,7 @@ future GitLab releases.**
 | `CI_BUILDS_DIR`                         | all    | 11.10  | Top-level directory where builds are executed. |
 | `CI_CONCURRENT_ID`                      | all    | 11.10  | Unique ID of build execution within a single executor. |
 | `CI_CONCURRENT_PROJECT_ID`              | all    | 11.10  | Unique ID of build execution within a single executor and project. |
-| `CI_COMMIT_BEFORE_SHA`                  | 11.2   | all    | The previous latest commit present on a branch before a push request. |
+| `CI_COMMIT_BEFORE_SHA`                  | 11.2   | all    | The previous latest commit present on a branch before a push request. Only populated when there is a merge request associated with the pipeline. |
 | `CI_COMMIT_DESCRIPTION`                 | 10.8   | all    | The description of the commit: the message without first line, if the title is shorter than 100 characters; full message in other case. |
 | `CI_COMMIT_MESSAGE`                     | 10.8   | all    | The full commit message. |
 | `CI_COMMIT_REF_NAME`                    | 9.0    | all    | The branch or tag name for which project is built |