docs: document legacy read API rate limits

Author: maxdeichmann

content/faq/all/api-limits.mdx

@@ -34,8 +34,11 @@ Rate limits are applied per-organization.
 | **Prompts**. GET APIs used to fetch prompts for use in applications.                                                 | No limit           | No limit           | No limit            |
 | **Metrics**. GET APIs used to fetch analytics data, e.g. [metrics api](/docs/analytics/metrics-api).                 | 100 req/day   | 200 req/day   | 2000 req/day   |
 | **Datasets**. APIs used to work with [datasets and experiments](/docs/datasets).                                     | 100 req/min   | 200 req/min   | 1,000 req/min  |
+| **Legacy read APIs**. Older trace, observation, and session read endpoints, listed below.                            | 15 req/min    | 30 req/min    | 100 req/min    |
 | **API**. All other APIs.                                                                                             | 30 req/min    | 100 req/min   | 1,000 req/min  |
 
+The legacy read APIs are `GET /api/public/traces`, `GET /api/public/traces/{traceId}`, `GET /api/public/observations`, `GET /api/public/observations/{observationId}`, `GET /api/public/sessions`, and `GET /api/public/sessions/{sessionId}`.
+
 On the Enterprise plan, you can request custom rate limits by contacting [support](/support).
 
 **Rate limit response**