chore(doc): clarify service tier default value (#908)

.stats.yml

@@ -1,2 +1,2 @@
 configured_endpoints: 64
-openapi_spec_url: https://storage.googleapis.com/stainless-sdk-openapi-specs/openai-8fe357c6b5a425d810d731e4102a052d8e38c5e2d66950e6de1025415160bf88.yml
+openapi_spec_url: https://storage.googleapis.com/stainless-sdk-openapi-specs/openai-3a69e1cc9e1efda3fb82d0fb35961749f886a87594dae9d8d2aa5c60f157f5d2.yml

src/resources/chat/completions.ts

@@ -818,7 +818,8 @@ export interface ChatCompletionCreateParamsBase {
    *
    * - If set to 'auto', the system will utilize scale tier credits until they are
    *   exhausted.
-   * - If set to 'default', the request will be processed in the shared cluster.
+   * - If set to 'default', the request will be processed using the default service
+   *   tier with a lower uptime SLA and no latency guarentee.
    *
    * When this parameter is set, the response body will include the `service_tier`
    * utilized.