Document how the executing kernel is chosen

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 '' ?