Added further details to the CI_PROJECT_DIR variable highlighting a dependency...

Added further details to the CI_PROJECT_DIR variable highlighting a dependency to the Runner builds_dir configuration option.

doc/ci/variables/README.md

@@ -85,7 +85,7 @@ future GitLab releases.**
 | **CI_PIPELINE_SOURCE**                    | 10.0   | all    | Indicates how the pipeline was triggered. Possible options are: `push`, `web`, `trigger`, `schedule`, `api`, and `pipeline`. For pipelines created before GitLab 9.5, this will show as `unknown` |
 | **CI_PIPELINE_TRIGGERED**                 | all    | all    | The flag to indicate that job was [triggered] |
 | **CI_PIPELINE_URL**                       | 11.1   | 0.5    | Pipeline details URL |
-| **CI_PROJECT_DIR**                        | all    | all    | The full path where the repository is cloned and where the job is run |
+| **CI_PROJECT_DIR**                        | all    | all    | The full path where the repository is cloned and where the job is run. Note that if the GitLab Runner `builds_dir` parameter is set it will have an impact on this. See [GitLab Runner - Advanced Configuration](https://docs.gitlab.com/runner/configuration/advanced-configuration.html#the-runners-section). |
 | **CI_PROJECT_ID**                         | all    | all    | The unique id of the current project that GitLab CI uses internally |
 | **CI_PROJECT_NAME**                       | 8.10   | 0.5    | The project name that is currently being built (actually it is project folder name) |
 | **CI_PROJECT_NAMESPACE**                  | 8.10   | 0.5    | The project namespace (username or groupname) that is currently being built |