Additional Javadoc explaining why RunnerStack.peek might return null.

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

@@ -53,6 +53,8 @@ import javax.annotation.Nonnull;
  * This class defines a few well-known check point instances. plugins can define
  * their additional check points for their own use.
  *
+ * <p>Note that not all job types support checkpoints.
+ *
  * <h2>Example</h2>
  * <p>
  * {@link JUnitResultArchiver} provides a good example of how a {@link Recorder} can

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

@@ -56,6 +56,10 @@ final class RunnerStack {
 
     /**
      * Looks up the currently running build, if known.
+     * <p>While most {@link Run} implementations do add a {@link RunExecution} to the stack for the duration of the build,
+     * those which have a different implementation of {@link Run#run} (or which do additional work after {@link Run#execute} completes)
+     * may not consistently or ever keep an execution on the stack.
+     * In such cases this method will return null, meaning that {@link CheckPoint#block(BuildListener, String)} and {@link CheckPoint#report} will do nothing.
      * @return a running build, or null if one has not been recorded
      */
     synchronized @CheckForNull RunExecution peek() {