Add missing javadoc comment.

Author: raphw

byte-buddy-dep/src/main/java/net/bytebuddy/build/Plugin.java

@@ -3713,6 +3713,9 @@ class ForFolder implements Target, Sink {
                  */
                 private final File folder;
 
+                /**
+                 * {@code true} if retained files should be linked and not copied.
+                 */
                 private final boolean link;
 
                 /**