BatchRefUpdate: Document when getPushOptions is null

Change-Id: I4cccda0ec3a8598edb723dc49101a16d603d1e82

org.eclipse.jgit/src/org/eclipse/jgit/lib/BatchRefUpdate.java

@@ -58,6 +58,7 @@ import java.util.HashSet;
 import java.util.List;
 import java.util.concurrent.TimeoutException;
 
+import org.eclipse.jgit.annotations.Nullable;
 import org.eclipse.jgit.internal.JGitText;
 import org.eclipse.jgit.lib.RefUpdate.Result;
 import org.eclipse.jgit.revwalk.RevWalk;
@@ -323,9 +324,11 @@ public class BatchRefUpdate {
 	/**
 	 * Gets the list of option strings associated with this update.
 	 *
-	 * @return pushOptions
+	 * @return push options that were passed to {@link #execute}; prior to calling
+	 *         {@link #execute}, always returns null.
 	 * @since 4.5
 	 */
+	@Nullable
 	public List<String> getPushOptions() {
 		return pushOptions;
 	}