Minor edits based on review comments

core/src/main/java/hudson/model/AbstractProject.java

@@ -1180,6 +1180,8 @@ public abstract class AbstractProject<P extends AbstractProject<P,R>,R extends A
 
     /**
      * Let the identity determined by {@link ProjectAuthenticator}.
+     *
+     * @since 1.520
      */
     public Authentication getIdentity() {
         return ProjectAuthenticatorConfiguration.get().authenticate(this);

core/src/main/java/jenkins/security/ProjectAuthenticator.java

@@ -7,6 +7,8 @@ import hudson.model.AbstractProject;
 import hudson.security.ACL;
 import org.acegisecurity.Authentication;
 
+import javax.annotation.CheckForNull;
+
 /**
  * Extension point to run {@link AbstractBuild}s under a specific identity for better access control.
  *
@@ -28,7 +30,7 @@ public abstract class ProjectAuthenticator extends AbstractDescribableImpl<Proje
      *      the executor. If everything fails, fall back to the historical behaviour of
      *      {@link ACL#SYSTEM}.
      */
-    public abstract Authentication authenticate(AbstractProject<?,?> project);
+    public abstract @CheckForNull Authentication authenticate(AbstractProject<?,?> project);
 
     @Override
     public ProjectAuthenticatorDescriptor getDescriptor() {

core/src/main/java/jenkins/security/ProjectAuthenticatorConfiguration.java

@@ -17,6 +17,7 @@ import java.io.IOException;
  * Show the {@link ProjectAuthenticator} configurations on the system config page.
  *
  * @author Kohsuke Kawaguchi
+ * @since 1.520
  */
 @Extension
 public class ProjectAuthenticatorConfiguration extends GlobalConfiguration {