Summary

Updates the Billing developer docs to cover the default-on Budget alerts release. Adds a "Default budget alert" section to the Budget alerts page, clarifies that alerts only count usage-based spend (not recurring subscription fees like the Workers Paid plan fee), and updates the Usage-based billing page to mention default alerts.

Companion changelog PR

This is the docs counterpart to the changelog post in PR #31467.

Changes

src/content/docs/billing/manage/budget-alerts.mdx

• New section ## Default budget alert between the intro and "Create a budget alert." Explains the $10 default threshold, cycle-start activation, that the alert is informational only, and that existing user-set alerts are untouched.
• Clarifies scope in the new section and in "How budget alerts work": only spend on usage-based products counts toward the threshold; recurring subscription fees (Workers Paid plan fee, monthly plan charges) are excluded.
• New bullet in "How budget alerts work" confirming default alerts behave the same as user-created alerts.

src/content/docs/billing/understand/usage-based-billing.mdx

• One sentence updated in "Monitor your usage" to note that eligible accounts now receive a default budget alert automatically, with the $10 threshold and cycle-start activation.

Validation

• astro check: 0 errors, 0 warnings on changed files
• Local preview verified at /billing/manage/budget-alerts/ and /billing/understand/usage-based-billing/

Review

Per the docs review process, will route for PM review.