Update WebClient builder

Leave only one static, no-arg build() method for access to a Builder.

URI-related preferences are now exposed on the builder itself.

Improve Javadoc with base URI examples.

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/DefaultWebClientTests.java

@@ -99,7 +99,7 @@ public class DefaultWebClientTests {
 
 
 	private WebClient.Builder builder() {
-		return WebClient.builder("/base").exchangeFunction(this.exchangeFunction);
+		return WebClient.builder().baseUrl("/base").exchangeFunction(this.exchangeFunction);
 	}
 
 	private ClientRequest<?> verifyExchange() {