docs: update concurrency tier thresholds (#66)

* docs: add 1000 usd concurrent limit tier

* docs: clarify concurrency tier unlock

Author: Kilerd

developer-guide/models-pricing/pricing-and-rate-limits.mdx

@@ -79,8 +79,13 @@ These limits help us ensure fair usage and maintain service quality for all user
 |------------|--------------------|---------------------|
 | Starter    | < $100 paid        | 5 requests          |
 | Elevated   | ≥ $100 paid        | 15 requests         |
+| High Volume | ≥ $1,000 paid     | 50 requests         |
 | Enterprise | Custom             | Custom limits       |
 
+<Tip>
+    Concurrency tiers unlock as soon as your total prepaid amount reaches the threshold. You do not need to spend the full balance first. If your workload needs a higher concurrency tier, you can top up in advance to unlock the next tier immediately.
+</Tip>
+
 <Note>
     Please reach out to our team to enable enterprise volume pricing, rate limits, and billing.
 </Note>