WOW, finally!

the trick is in {project:content.xdocs}. If you don't specify it, you'll get the error of non existing resource... If you specify it, the xml with listing of files in directory appears as it is the most obvious thing, it ever does... and how can I expect it will be not working? ;)
<map:generate type="directory" src="{project:content.xdocs}/my">  ... ok

Thanks for help,
Mirek