Update pipelines docs to clarify when full Jest jobs run

What does this MR do and why?

This MR clarifies when "full" Jest jobs run in pipelines based on the latest rule changes: https://gitlab.com/gitlab-org/gitlab/-/blob/c8ff28ea6cf27f81b792c2a535f4120cde925337/.gitlab/ci/rules.gitlab-ci.yml#L938-955.

Suggested here: #382740 (closed)

Thanks to @godfat-gitlab for the suggestions!

MR acceptance checklist

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Related to #382740 (closed)