Thanks for the reply, but then if I want to run format, it will always also run document, that's not what I want.

@jakubmazanec if you do need to ensure that document has been run before format, the dependency is the correct way to do it, you do in fact have a dependency. To avoid rerunning document unnecessarily, take advantage of caching

That's not what I need. I need to ensure that format task is run after document task. The thing is,format is a task that is sometimes used on its own - you simply want to ensure your files are formatted correctly - so it can't have any dependencies (I don't want to run document needlessly, and caching will not help with that).
document task (i.e. generating documentation) produces files that are sometimes not formatted correctly, that's the reason why I would like that format is run after document automatically. I could make the document task itself contain commands that format the files, I just thought that it would be nice if I could reuse existing task. But it seems that we can describe the realtion between some two tasks only in one direction (via dependsOn property), but not the other way around, i.e. what task is depending on this task.

with would not help your use case as with doesn't provide any ordering so the formatter could run before the generation finishes.