Merge pull request #3666 from oltruong/JENKINS-53847

[JENKINS-53847] Improve JavaDoc for Util.fixNull (Hacktoberfest)

core/src/main/java/hudson/Util.java

@@ -1200,6 +1200,7 @@ public class Util {
 
     /**
      * Convert {@code null} to a default value.
+     * @param defaultValue Default value. It may be immutable or not, depending on the implementation.
      * @since TODO
      */
     @Nonnull
@@ -1227,21 +1228,57 @@ public class Util {
         return fixEmpty(s.trim());
     }
 
+    /**
+     *
+     * @param l list to check.
+     * @param <T>
+     *     Type of the list.
+     * @return
+     *     {@code l} if l is not {@code null}.
+     *     An empty <b>immutable list</b> if l is {@code null}.
+     */
     @Nonnull
     public static <T> List<T> fixNull(@CheckForNull List<T> l) {
         return fixNull(l, Collections.<T>emptyList());
     }
 
+    /**
+     *
+     * @param l set to check.
+     * @param <T>
+     *     Type of the set.
+     * @return
+     *     {@code l} if l is not {@code null}.
+     *     An empty <b>immutable set</b> if l is {@code null}.
+     */
     @Nonnull
     public static <T> Set<T> fixNull(@CheckForNull Set<T> l) {
         return fixNull(l, Collections.<T>emptySet());
     }
 
+    /**
+     *
+     * @param l collection to check.
+     * @param <T>
+     *     Type of the collection.
+     * @return
+     *     {@code l} if l is not {@code null}.
+     *     An empty <b>immutable set</b> if l is {@code null}.
+     */
     @Nonnull
     public static <T> Collection<T> fixNull(@CheckForNull Collection<T> l) {
         return fixNull(l, Collections.<T>emptySet());
     }
 
+    /**
+     *
+     * @param l iterable to check.
+     * @param <T>
+     *     Type of the iterable.
+     * @return
+     *     {@code l} if l is not {@code null}.
+     *     An empty <b>immutable set</b> if l is {@code null}.
+     */
     @Nonnull
     public static <T> Iterable<T> fixNull(@CheckForNull Iterable<T> l) {
         return fixNull(l, Collections.<T>emptySet());