Add examples for multiple jobs in needs:project

What does this MR do and why?

Describe in detail what your merge request does and why.

A customer brought to our attention that the .gitlab-ci.yml keyword reference mentions "Use needs:project to download artifacts from up to five jobs in other pipelines." However, there was no example provided. This MR adds an example and some additional clarifying language.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

This was tested by @rkelly here.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.