Better documentation/example about `BatchController`

## Description

I wanted to make bunch of systems conditional (eg. running them only when the game is not paused). I thought I'd use the `DispatcherBuilder::with_batch` and decide if I want to call a sub-dispatcher in the controller.

But I'm not entirely sure how it is supposed to work. The `Safety` section of the `create` method isn't really clear on what must be uphold. What is „correctly constructed BatchAccessor“?

Furthermore, it references `examples/batch_dispatching.rs`, which doesn't exist (at least not on master).

So I'd like to ask if it's possible to describe how this thing works in some better way/add the missing example. Or am I trying to use the wrong tool for the job? Shouldn't something like enabling/disabling bunch of systems be possible without using unsafe?

I'm trying to wrap my head around how I would use the BatchController.

(I'm not sure if this belongs more into the feature or bug category, to be honest)