Docs feedback: Fix confusing description duplication for CI_MERGE_REQUEST_IID and CI_MERGE_REQUEST_ID

Doc: https://docs.gitlab.com/ee/ci/variables/predefined_variables.html Fix description of "CI_MERGE_REQUEST_IID" to include 'per project scope' and "CI_MERGE_REQUEST_ID" to be global. This will align documentation to actual behavior and spirit of issue gitlab-foss!23398 (merged)

| `CI_MERGE_REQUEST_ID`                         | 11.6   | all    | The ID of the merge request GitLab CI uses internally
(global) if [the pipelines are for merge requests](../merge_request_pipelines/index.md). Available only if `only: [merge_requests]` is used and the merge request is created.                                                                                                                                                           |
| `CI_MERGE_REQUEST_IID`                        | 11.6   | all    | The IID of the merge request scoped to project if [the pipelines are for merge requests](../merge_request_pipelines/index.md). Available only if `only: [merge_requests]` is used and the merge request is created.  

This would be consistent with CI_PIPELINE_ID and CI_PIPELINE_IID descriptions.