text: Title: Document how the executing kernel is chosen
  Body: Hi there,
  
  I recently reinstalled my kernels and suddenly, my tests were no longer passing due to the wrong kernel being used... 
  
  I managed to fix the problem but I think it would be nice in the docs to:
  - Tell that by default, the executing kernel is the one named `python3`
  - Show that this can be changed by setting `kernel_name` in the decorator
  - And perhaps that the kernel can be set from the metadata of the notebook by setting `kernel_name` to `''` ?