Add OAuth2 server implementation section to docs

As auto-configuration for Spring Security OAuth has been removed
from Spring Boot 2.0 and Spring Security 5 doesn't have OAuth
2.0 Authorization / Resource Server support yet, it has not
been obvious at all how to implement an OAUth 2.0 server with
Spring Boot 2.0.

For that reason, this new section briefly explains the current
temporary situation and points to the spring-security-oauth2-autoconfigure
module that restores the auto-configuration support for OAuth
2.0 Authorization and Resource Servers.

Closes gh-12491

spring-boot-project/spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -3193,6 +3193,17 @@ In other words, the two configurations in the following example use the Google p
 
 
 
+[[boot-features-security-oauth2-server]]
+==== Server
+Currently, Spring Security does not provide support for implementing an OAuth 2.0
+Authorization Server or Resource Server. However, this functionality is available from
+the https://projects.spring.io/spring-security-oauth/[Spring Security OAuth] project,
+which will eventually be superseded by Spring Security completely. Until then, you can
+use the `spring-security-oauth2-autoconfigure` module to easily set up an OAuth 2.0 server;
+see its https://docs.spring.io/spring-security-oauth2-boot[documentation] for instructions.
+
+
+
 [[boot-features-security-actuator]]
 === Actuator Security
 For security purposes, all actuators other than `/health` and `/info` are disabled by