Rework linking to Spring MVC Async support vs WebFlux section

The link was previously named "Compared to WebFlux", which is easy to
mix up with the various links to equivalent sections in the WebFlux
chapter. Here the links point to a small section comparing the Servlet
Async API to the WebFlux stack from a high perspective.

In this commit we eliminate most of these links, except at the
beginning of the Asynchronous section. We also add a small mention of
the Servlet configuration in the comparison paragraphs, since the
Configuring section is the one furthest from the comparison paragraphs
that used to have a link to it.

Closes spring-projectsgh-29694

framework-docs/src/docs/asciidoc/web/webmvc.adoc

@@ -4366,7 +4366,6 @@ capital letters of the class and the method name (for example, the `getThing` me
 
 [[mvc-ann-async]]
 == Asynchronous Requests
-[.small]#<<mvc-ann-async-vs-webflux, Compared to WebFlux>>#
 
 Spring MVC has an extensive integration with Servlet asynchronous request
 <<mvc-ann-async-processing,processing>>:
@@ -4379,11 +4378,10 @@ return value.
 * Controllers can use reactive clients and return
 <<mvc-ann-async-reactive-types, reactive types>> for response handling.
 
-
+For an overview of how this differs from Spring WebFlux, see the <<mvc-ann-async-vs-webflux>> section below.
 
 [[mvc-ann-async-deferredresult]]
 === `DeferredResult`
-[.small]#<<mvc-ann-async-vs-webflux, Compared to WebFlux>>#
 
 Once the asynchronous request processing feature is <<mvc-ann-async-configuration, enabled>>
 in the Servlet container, controller methods can wrap any supported controller method
@@ -4425,7 +4423,6 @@ example, in response to an external event (JMS message), a scheduled task, or ot
 
 [[mvc-ann-async-callable]]
 === `Callable`
-[.small]#<<mvc-ann-async-vs-webflux, Compared to WebFlux>>#
 
 A controller can wrap any supported return value with `java.util.concurrent.Callable`,
 as the following example shows:
@@ -4455,7 +4452,6 @@ The return value can then be obtained by running the given task through the
 
 [[mvc-ann-async-processing]]
 === Processing
-[.small]#<<mvc-ann-async-vs-webflux, Compared to WebFlux>>#
 
 Here is a very concise overview of Servlet asynchronous request processing:
 
@@ -4533,7 +4529,7 @@ methods for timeout and completion callbacks.
 
 
 [[mvc-ann-async-vs-webflux]]
-==== Compared to WebFlux
+==== Async Spring MVC compared to WebFlux
 
 The Servlet API was originally built for making a single pass through the Filter-Servlet
 chain. Asynchronous request processing lets applications exit the Filter-Servlet chain
@@ -4560,6 +4556,8 @@ types in controller method arguments (for example, `@RequestBody`, `@RequestPart
 nor does it have any explicit support for asynchronous and reactive types as model attributes.
 Spring WebFlux does support all that.
 
+Finally, from a configuration perspective the asynchronous request processing feature must be
+<<mvc-ann-async-configuration, enabled at the Servlet container level>>.
 
 
 [[mvc-ann-async-http-streaming]]
@@ -4809,7 +4807,6 @@ that have a built-in heartbeat mechanism.
 
 [[mvc-ann-async-configuration]]
 === Configuration
-[.small]#<<mvc-ann-async-vs-webflux, Compared to WebFlux>>#
 
 The asynchronous request processing feature must be enabled at the Servlet container level.
 The MVC configuration also exposes several options for asynchronous requests.