docs(slo-alerts): request rate drop baseline needs the source's history

The request rate drop docs say the baseline is complete as soon as the rules are deployed. That holds only when the SLI's source aggregation already has history for every offset.

sli_aggregations:pipelines_finished_total:rate_5m only has data in gprd since 2026-11-02 19:05, so the drop alert in !12057 (merged) has no baseline until 2026-11-09 (and cannot fire), and a full 3-week baseline from 2026-11-23. This corrects the docs and the code comment that make the claim. No generated rules change.