[FIXED JENKINS-19947] Restore historical undocumented behavior of a top-level entry in a ZIP.

Noting that various DirScanner implementations handle this inconsistently.

core/src/main/java/hudson/FilePath.java

@@ -392,7 +392,7 @@ public final class FilePath implements Serializable {
      *
      * @param glob
      *      Ant style glob, like "**/*.xml". If empty or null, this method
-     *      works like {@link #createZipArchive(OutputStream)}
+     *      works like {@link #createZipArchive(OutputStream)}, inserting a top-level directory into the ZIP.
      *
      * @since 1.315
      */