docs: update billing API report data limitations for public preview

Expands granularity options from hourly-only to hourly, daily, and monthly,
with corresponding date range support per granularity level.

Author: jsundai

docs/cloud/billing-and-usage/billing-api.mdx

@@ -42,7 +42,10 @@ Billing report generation is **asynchronous**. You initiate report creation, the
 
 ## Report data limitations
 
-For pre-release, reports can be generated with hourly granularity only, for the current billing month, and the previous billing month.
+Reports can be generated with hourly, daily and monthly granularity.
+Hourly for the current billing month, and the previous billing month.
+Daily for the current billing month, and previous three billing months.
+Monthly for the current billing month, and previous twelve billing months.
 
 ## Allowed date ranges