MultipartBodyBuilder Javadoc update

Closes gh-24031

spring-web/src/main/java/org/springframework/http/client/MultipartBodyBuilder.java

@@ -39,11 +39,21 @@ import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
 
 /**
- * Builder for the body of a multipart request, producing
- * {@code MultiValueMap<String, HttpEntity>}, which can be provided to the
- * {@code WebClient} through the {@code body} method.
+ * Prepare the body of a multipart request, resulting in a
+ * {@code MultiValueMap<String, HttpEntity>}. Parts may be concrete values or
+ * via asynchronous types such as Reactor {@code Mono}, {@code Flux}, and
+ * others registered in the
+ * {@link org.springframework.core.ReactiveAdapterRegistry ReactiveAdapterRegistry}.
  *
- * Examples:
+ * <p>This builder is intended for use with the reactive
+ * {@link org.springframework.web.reactive.function.client.WebClient WebClient}.
+ * For multipart requests with the {@code RestTemplate}, simply create and
+ * populate a {@code MultiValueMap<String, HttpEntity>} as shown in the Javadoc for
+ * {@link org.springframework.http.converter.FormHttpMessageConverter FormHttpMessageConverter}
+ * and in the
+ * <a href="https://docs.spring.io/spring/docs/current/spring-framework-reference/integration.html#rest-template-multipart">reference docs</a>.
+ *
+ * <p>Below are examples of using this builder:
  * <pre class="code">
  *
  * // Add form field