Closes gh-1967

I edited for spelling, puncutation, grammar, usage, and corporate voice. I also added a bunch of cross-references and links to the Javadoc.

I also corrected the rendering problems that I had accidentally created in previous commits (because I was adjusting headings and adding callouts to listings.

Issue: SPR-17241

This commit also fixes a few typos and broken links.

This commit also fixes some broken links and typos.

I edited for spelling, punctuation, grammar, usage,
and corporate voice. I also added links and cross-references.

I edited the Data Access chapter for spelling, punctuation,
grammar, usage, corporate voice, and clarity.

I also added links and cross-references.

I edited for spelling, punctuation, grammar, usage,
and corporate voice.

I also added cross-references and links to the Javadoc.

I edited for the usual stuff: spelling, punctuation,
grammar, formatting, usage, and voice.

Before this commit, Kotlin inner class nested configuration
handling thrown an IndexOutOfBoundsException due to bogus filtering
of its constructor parameter reference to an instance of the outer
class.

This commit keep constructor parameter of type INSTANCE in order to
throw a more meaningful NoSuchBeanDefinitionException.

Issue: SPR-17222

1. Rename globalResources to useGlobalResources.
2. Use of global resources is mutually exlusive with explicit config.
3. Allow Consumer<HttpResources> to configure global resources.
4. Allow ConnectionProvider + LoopResources Supplier to customize
   creation and initialization.
5. Do not manage externally provided ConnectionProvider + LoopResources
   instances.

Issue: SPR-17243

I edited for spelling, punctuation, grammar, usage, and corporate voice. I also added cross-references and links to the Javadoc in a number of places.

Closes gh-1934

JettyResourceFactory, similar to ReactorResourceFactory, allows
to share resources (Executor, ByteBufferPool, Scheduler) between
Jetty clients and servers.

Issue: SPR-17179

Issue: SPR-17189

This reverts commit 7e917335.

This commit handles "empty" cases for `ResponseEntity` controller
handler return types when wrapped with a `java.util.Optional` in Spring
MVC or a single `Publisher` like `Mono`.

Given the following example for Spring MVC:

```
@GetMapping("/user")
public Optional<ResponseEntity<User>> fetchUser() {
	Optional<User> user = //...
	return user.map(ResponseEntity::ok);
}
```

If the resulting `Optional` is empty, Spring MVC will infer a
`ResponseEntity` with an empty body and a 404 HTTP response status.

The same reasoning is applied to Spring WebFlux with Publisher types:

```
@GetMapping("/user")
public Mono<ResponseEntity<User>> fetchUser() {
	Mono<User> user = //...
	return user.map(ResponseEntity::ok);
}
```

This feature is only valid for `HttpEntity` return types and does not
apply to `@ResponseBody` controller handlers.

Issue: SPR-13281

Issue: SPR-17180
Issue: SPR-17182

Rename "Builder" sub-section to "Configuration" and move it in the
beginning before all others since it explains how to create a client
in the first place.

Update content on Reactor Netty connector based on the API in 0.8 and
specifically address Reactor Netty resources and lifecycle.

Issue: SPR-16963

Issue: SPR-17167

Issue: SPR-17145

Issue: SPR-17143

Forwarded headers are now processed before ServerWebExchange is created
through ForwardedHeaderTransformer which has the same logic as the
ForwardedHeaderFilter but works on the request only.

ForwardedHeaderFilter is deprecated as of 5.1 but if registered it is
removed from the list of filters and ForwardedHeaderTransformer is used
instead.

Issue: SPR-17072

Issue: SPR-17104

Issue: SPR-17032