Add documentation for DPoP support

Closes gh-2009

oauth2-authorization-server/src/main/java/org/springframework/security/oauth2/server/authorization/authentication/DPoPProofVerifier.java

@@ -26,8 +26,13 @@ import org.springframework.security.oauth2.jwt.JwtDecoderFactory;
 import org.springframework.util.StringUtils;
 
 /**
+ * A verifier for DPoP Proof {@link Jwt}'s.
+ *
  * @author Joe Grandja
  * @since 1.5
+ * @see DPoPProofJwtDecoderFactory
+ * @see <a target="_blank" href="https://datatracker.ietf.org/doc/html/rfc9449">RFC 9449
+ * OAuth 2.0 Demonstrating Proof of Possession (DPoP)</a>
  */
 final class DPoPProofVerifier {
 

oauth2-authorization-server/src/main/java/org/springframework/security/oauth2/server/authorization/token/OAuth2TokenContext.java

@@ -45,6 +45,7 @@ import org.springframework.util.Assert;
 public interface OAuth2TokenContext extends Context {
 
 	/**
+	 * The key used for the DPoP Proof {@link Jwt} (if available).
 	 * @since 1.5
 	 */
 	String DPOP_PROOF_KEY = Jwt.class.getName().concat(".DPOP_PROOF");