doc/acquire-additional-files.txt: Mention apt-helper cat-file

People should know that this is exists.

doc/acquire-additional-files.txt

@@ -88,7 +88,8 @@ Additional optional properties:
   this option to false to avoid problems with users setting the option
   globally. On the other hand, if you set it to true or don't set it you
   have to ensure your front-end can deal with all compressed fileformats
-  supported by apt (libapt users can e.g. use FileFd).
+  supported by apt (libapt users can e.g. use FileFd, others can use
+  the cat-file command of /usr/lib/apt/apt-helper).
 * flat{MetaKey,Description}: APT supports two types of repositories:
   dists-style repositories which are the default and by far the most
   common which are named after the fact that the files are in an