docs(billing): scope the usage_log workspace index note to created_at windows (#9479)

* docs(billing): scope the usage_log workspace index note to created_at windows

* docs(billing): limit the workspace index note to the reads it covers

Author: waleedlatif1

apps/sim/lib/billing/core/usage-analytics-queries.ts

@@ -27,8 +27,11 @@ import type { DbClient } from '@/lib/db/types'
  * and the member/source breakdowns are index-only. `workspace_id`, `workflow_id`,
  * `description`, `metadata`, and `execution_id` are NOT in it, so the remaining
  * reads heap-fetch per row — which is why they are issued only for the dimension
- * actually being viewed. A read narrowed to one workspace runs on
- * `usage_log_workspace_created_at_cost_idx` instead, which also covers `workflow_id`.
+ * actually being viewed. Narrowed to one workspace over a `created_at` window (a range
+ * or a reporting period), the totals, the series, and the member, source and workflow
+ * breakdowns are index-only on `usage_log_workspace_created_at_cost_idx` instead, which
+ * also covers `workspace_id` and `workflow_id`; the model breakdowns still heap-fetch,
+ * and a Stripe-period drill-down matches the period stamps, which that index lacks.
  */
 
 export interface UsageTimeSeriesRow {