[Billing] Document product family filter and multi-subscription support (#31762)

Author: ryan-noel-2258

src/content/docs/billing/manage/billable-usage.mdx

@@ -50,14 +50,27 @@ Below the chart, a sortable table breaks down usage by product for the full bill
 | **Billable usage** | Usage that exceeds the free tier and will be charged.                                                                                                    |
 | **Usage cost**     | Cumulative cost for the product in the selected billing period.                                                                                          |
 
-Use the **product filter** at the top of the page to narrow the chart and table to specific products.
+## Filter usage by product family or product
+
+Use the filters sidebar to narrow the dashboard to a subset of products.
+
+- **Product family** — Group products by family (for example, Workers or R2) to compare costs across related usage metrics.
+- **Product** — Filter to specific usage metrics. The list of available products narrows based on the families you select.
+
+Applied filters scope the cost breakdown chart, product usage table, and summary totals to your selection. On mobile, the sidebar opens as a drawer with an **Apply** action. Select **Reset** to clear all filters.
 
 ## Select a billing period
 
 By default, the dashboard shows data for your current billing period. Use the date picker to view a previous billing period.
 
 Usage data is aligned to your billing cycle, not the calendar month. Your billing period start date is determined by the first purchase date on your account.
 
+## Switch between subscriptions
+
+In rare cases, an account has more than one usage-based subscription — usually because a previous subscription was replaced. If this applies to your account, a **Subscription** filter appears in the sidebar, with each subscription labeled by its start date.
+
+Selecting a different subscription scopes the chart, product usage table, and available billing periods to that subscription's data. Each subscription has its own billing cycle.
+
 ## Data alignment with your invoice
 
 The dashboard reads from the same data source that generates your monthly invoice.