Update What's New for 5.6

docs/manual/src/docs/asciidoc/_includes/about/whats-new.adoc

@@ -13,4 +13,20 @@ Below are the highlights of the release.
 * SAML 2.0 Service Provider
 
 ** Added https://github.com/spring-projects/spring-security/pull/9483[SAML 2.0 Single Logout Support]
-** Added https://github.com/spring-projects/spring-security/pull/10060[Saml2AuthenticationRequestRepository]
+** Added https://github.com/spring-projects/spring-security/pull/10060[Saml2AuthenticationRequestRepository]
+
+* OAuth 2.0 Login
+
+** Added https://github.com/spring-projects/spring-security/pull/10041[`Converter` for `Authentication` result]
+
+* OAuth 2.0 Client
+
+** Improved https://github.com/spring-projects/spring-security/pull/9791[client credentials] encoding
+** Improved https://github.com/spring-projects/spring-security/pull/9779[access token response] parsing
+** Added https://github.com/spring-projects/spring-security/pull/10155[custom grant types support] for authorization requests
+
+[[whats-new-webflux]]
+=== WebFlux
+
+** Improved https://github.com/spring-projects/spring-security/pull/9791[client credentials] encoding
+** Added https://github.com/spring-projects/spring-security/pull/10131[custom headers support] for access token requests