Document effect of nested batching

[mathieuprog]

We could document what the effect is of having a batch call inside another batch. If I'm not wrong, it just executes the function without batch-specific handling as that has already been taken care of in the top batch call. If this is correct:

If batch is called inside another batch, the function is simply executed, without [... whatever batch does]

https://www.solidjs.com/docs/latest#batch