Polish + minor HttpHandler refactoring

CompositeHttpHandler is public and called ContextPathCompositeHandler.

Also an overhaul of the Javadoc on HttpHandler, WebHttpHandlerAdapter,
and ContextPathCompositeHandler.

spring-webflux/src/main/java/org/springframework/web/reactive/package-info.java

@@ -1,4 +1,12 @@
 /**
- * Core interfaces and classes for Spring Web Reactive.
+ * Top-level package for the {@code spring-webflux} module that contains
+ * {@link org.springframework.web.reactive.DispatcherHandler}, the main entry
+ * point for WebFlux server endpoint processing including key contracts used to
+ * map requests to handlers, invoke them, and process the result.
+ *
+ * <p>The module provides two programming models for reactive server endpoints.
+ * One based on annotated {@code @Controller}'s and another based on functional
+ * routing and handling. The module also contains a functional, reactive
+ * {@code WebClient} as well as client and server, reactive WebSocket support.
  */
 package org.springframework.web.reactive;