Link to Spring Security OAuth2 reference docs

Update documentation to include a link to Spring Security OAuth2's
reference documentation.

Closes gh-6909

spring-boot-docs/src/main/asciidoc/index.adoc

@@ -37,6 +37,7 @@ Phillip Webb; Dave Syer; Josh Long; Stéphane Nicoll; Rob Winch; Andy Wilkinson;
 :spring-boot-maven-plugin-site: http://docs.spring.io/spring-boot/docs/{spring-boot-docs-version}/maven-plugin
 :spring-reference: http://docs.spring.io/spring/docs/{spring-docs-version}/spring-framework-reference/htmlsingle
 :spring-security-reference: http://docs.spring.io/spring-security/site/docs/{spring-security-docs-version}/reference/htmlsingle
+:spring-security-oauth2-reference: http://projects.spring.io/spring-security-oauth/docs/oauth2.html
 :spring-webservices-reference: http://docs.spring.io/spring-ws/docs/{spring-webservices-docs-version}/reference/htmlsingle
 :spring-javadoc: http://docs.spring.io/spring/docs/{spring-docs-version}/javadoc-api/org/springframework
 :spring-amqp-javadoc: http://docs.spring.io/spring-amqp/docs/current/api/org/springframework/amqp

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

@@ -2377,7 +2377,9 @@ explicitly configure the paths that you do want to override.
 [[boot-features-security-oauth2]]
 === OAuth2
 If you have `spring-security-oauth2` on your classpath you can take advantage of some
-auto-configuration to make it easy to set up Authorization or Resource Server.
+auto-configuration to make it easy to set up Authorization or Resource Server. For full
+details, see the {spring-security-oauth2-reference}[Spring Security OAuth 2 Developers
+Guide].