docs/ci/yaml: add warning about needs: & parallel: clobbering artifacts:

What does this MR do?

There is a "quirk" when using needs: to get the artifacts: of a job in a parallel: group.
Specifically, it is currently not possible to refer to a single job, but only its group, which has two consequences:

• The DAG creates a dependency on all the jobs in the group, regardless of how many are actually needed.
• All the artifacts of the group are clobbered.

This MR addresses this second issue, by documenting this behaviour, as well as a workaround.
Thanks to @mbobin for helping me figure this out 🙏

Related issues

The DAG part of this issue is covered by #254821 (closed), and is not affected by this MR.

#296909 (closed) covers the artifact loss, and therefore this MR:

Closes #296909 (closed)

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documentation
/cc @marcel.amirault