Clarify loss of DAG behavior with `needs:pipeline`

What does this MR do and why?

The reason why I opened this MR is because of a question raised by Support: internal slack link

Clarify that needs:pipeline (as well as other cross-pipeline artifacts downloads) can only be used to download artifacts and does not wait for the needed job to complete.

Example

java_build:
  stage: build
  trigger:
    include:
      - local: java/ci.yml
    strategy: depend
  variables:
    PARENT_PIPELINE_ID: $CI_PIPELINE_ID

docker-build:
  stage: build
  artifacts:
    paths: [images/]

# in java/ci.yml:
java-init:
  needs:
    - pipeline: $PARENT_PIPELINE_ID
      job: docker-build
      artifacts: true

Since docker-build and the trigger job java_build both run in parallel, there is a chance that java-init job attempts to download the artifacts from the parent pipeline that don't exist yet.

When needs:pipeline or needs:project are used (cross-pipeline artifacts download) the needs keyword looses the DAG behavior of waiting for the dependency to complete before running. This occurs because we don't support a cross-pipeline DAG behavior.

For cross-project artifacts download, the user must guarantee that the dependency docker-build completes before the other pipeline job java-init runs.

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