Issue: SPR-16040

Issue: SPR-16040

@EnableWebFlux bootstraps both annotated controllers and functional
endpoints, so we need to be more explicit about which parts of the
configuration apply to which.

Issue: SPR-16360

Issue: SPR-16360

Issue: SPR-16260

Cross-reference sections on HttpMessageReader/Writer and
HttpMessageConverter and improve the content.

Issue: SPR-16260

This commit allows CorsConfiguration#combine()
to differentiate permit default values set by
CorsConfiguration#applyPermitDefaultValues()
from values configured explicitly by the user.

Those permit default values will be overridden
by any user-provided ones while user-provided values
will be combined in an additive way, including
when "*" is specified.

Documentation has been improved accordingly.

Issue: SPR-15772

Issue: SPR-16300

Issue: SPR-16240

- Add "Processing" section (also replaces Advanced Customizations)
- Add information on out-of-the-box behavior
- Add more deails on @CrossOririn default configuratio
- Add cross-references between Spring MVC and WebFlux
- Polish

Access-Control-Allow-Credentials CORS header, used to
allow cookies with CORS requests, is not set to true
anymore by default when enabling CORS with
@CrossOrigin or global CORS configuration in order to
provide a more secured default CORS configuration.

The related allowCredentials property now requires to
be set to true explicitly in order to support cookies
with CORS requests.

Issue: SPR-16130

Issue: SPR-16156

Issue: SPR-16200

Issue: SPR-16199

Issue: SPR-15700

Issue: SPR-15700

Issue: SPR-16118

This commit introduces a new AbstractReactiveWebInitializer in
spring-web that relies on WebHttpHandlerBuilder to detect request
processing infrastructure beans from an ApplicationContext.

This eliminates the need to create a DispatcherHandler, since it is
expected to be a Spring bean, and as a result the initializers in
spring-webflux have been deprecated.

Issue: SPR-16144

Issue: SPR-16107

A new HTTP/2 support section has been added in the Spring MVC
documentation. Also, a dedicated wiki page is created for specific
container support information.

Issue: SPR-15821

The information was already parsed and available in a request attribute
but until now there were no argument resolvers to expose it.

Issue: SPR-16005

Issue: SPR-16087

Issue: SPR-16074

Issue: SPR-16041

Closes gh-1566

Easier to find (at the top level) and better summary of testing support.
Improve structure headings names.
Remove or update outdated content.