docs: revise readme docs about nested params (#143)

Author: stainless-app[bot]

README.md

@@ -373,7 +373,23 @@ ProjectCreateParams params = ProjectCreateParams.builder()
     .build();
 ```
 
-These can be accessed on the built object later using the `_additionalHeaders()`, `_additionalQueryParams()`, and `_additionalBodyProperties()` methods. You can also set undocumented parameters on nested headers, query params, or body classes using the `putAdditionalProperty` method. These properties can be accessed on the built object later using the `_additionalProperties()` method.
+These can be accessed on the built object later using the `_additionalHeaders()`, `_additionalQueryParams()`, and `_additionalBodyProperties()` methods.
+
+To set undocumented parameters on _nested_ headers, query params, or body classes, call the `putAdditionalProperty` method on the nested class:
+
+```java
+import com.braintrustdata.api.core.JsonValue;
+import com.braintrustdata.api.models.ProjectSettings;
+import com.braintrustdata.api.models.ProjectUpdateParams;
+
+ProjectUpdateParams params = ProjectUpdateParams.builder()
+    .settings(ProjectSettings.builder()
+        .putAdditionalProperty("secretProperty", JsonValue.from("42"))
+        .build())
+    .build();
+```
+
+These properties can be accessed on the nested built object later using the `_additionalProperties()` method.
 
 To set a documented parameter or property to an undocumented or not yet supported _value_, pass a [`JsonValue`](braintrust-java-core/src/main/kotlin/com/braintrustdata/api/core/JsonValue.kt) object to its setter: