The guidelines were originally created several years ago. This commit
does not fundamentally change anything but rather make the guidance
more compact.

The commit consolidates into into CONTRIBUTING.adoc information from
CONTRIBUTING-DOCUMENTATION.adoc and also the wiki page "The Lifecycle
of an Issue".

Issue: SPR-16108

Directly inlined hasLength implementations for proper nullability detection in IntelliJ, assuming a hasText checked value is never null afterwards. Since the JVM is going to do this at runtime anyway, this is effectively equivalent but more indicative for source code introspection algorithms.

Issue: SPR-15540

Issue: SPR-16107

Issue: SPR-16102

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

Issue: SPR-16077

Issue: SPR-16103

Issue: SPR-16091

Issue: SPR-16044

This commit exposes the ClientRequest's URL and HttpMethod fields via a
setter, so that they can be changed more easily in a request that was
created via ClientRequest.from(ClientRequest).

Issue: SPR-16093

Issue: SPR-16085

* pr/1575:
  Fix wrong javadoc example

Closes gh-1575

Issue: SPR-16092

Issue: SPR-16095

Issue: SPR-16095

Issue: SPR-15540

Issue: SPR-15756

Issue: SPR-16089

Issue: SPR-16085

https://github.com/spring-projects/spring-framework/wiki/Spring-Framework-Code-Style#line-wrapping

* pr/1570:
  Add missing "be" in Part's javadoc

Closes gh-1570

Issue: SPR-16090

Issue: SPR-16089

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