docs: Add info about onboarding config behavior (!3326)

mentioned in merge request das-floss/renovate!20

mentioned in merge request guardianproject-ops/terraform-aws-gitlab-runners!13

mentioned in merge request sorbet_typed/short_circuit!59 (merged)

mentioned in merge request datastelsel.nl/ci/renovate!15

mentioned in merge request sorbet_typed/props!158 (merged)

mentioned in merge request pipeline-components/org/renovate!266

mentioned in merge request hpctoolkit/renovate.ci!23

mentioned in merge request kadse-xyz/flux-control/shared!35

mentioned in merge request lama-corp/infra/services/renovate!21

mentioned in merge request FoxAmes/saytrs!29

mentioned in merge request kubitus-project/renovate-runner!371

mentioned in merge request ethima/dependency-management!160