[docs] Add missing JavaDocs (#6146)

wpiutil/src/main/java/edu/wpi/first/util/cleanup/CleanupPool.java

@@ -16,6 +16,9 @@ public class CleanupPool implements AutoCloseable {
   // state ArrayDeque is faster anyway.
   private final Deque<AutoCloseable> m_closers = new ArrayDeque<>();
 
+  /** Default constructor. */
+  public CleanupPool() {}
+
   /**
    * Registers an object in the object stack for cleanup.
    *

wpiutil/src/main/java/edu/wpi/first/util/cleanup/SkipCleanup.java

@@ -9,6 +9,7 @@ import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
 
+/** Attribute for telling JVM to skip object cleanup. */
 @Target(ElementType.FIELD)
 @Retention(RetentionPolicy.RUNTIME)
 public @interface SkipCleanup {}