Update reference with examples of multipart requests

Issue: SPR-16118

spring-test/src/main/java/org/springframework/test/web/reactive/server/WebTestClient.java

@@ -615,6 +615,17 @@ public interface WebTestClient {
 		/**
 		 * Set the body of the request to the given synchronous {@code Object} and
 		 * perform the request.
+		 * <p>This method is a convenient shortcut for:
+		 * <pre class="code">
+		 * .body(BodyInserters.fromObject(object))
+		 * </pre>
+		 * <p>The body can be a
+		 * {@link org.springframework.util.MultiValueMap MultiValueMap} to create
+		 * a multipart request. The values in the {@code MultiValueMap} can be
+		 * any Object representing the body of the part, or an
+		 * {@link org.springframework.http.HttpEntity HttpEntity} representing a
+		 * part with body and headers. The {@code MultiValueMap} can be built
+		 * conveniently using
 		 * @param body the {@code Object} to write to the request
 		 * @return a {@code Mono} with the response
 		 */