powerpc/smp: Document irq enable/disable after migrating IRQs

This code was until recently completely undocumented and even now the comment is
not very verbose.

We've already had one patch sent to remove the IRQ enable/disable because it's
"paradoxical and unnecessary". So document it thoroughly to save anyone else
from puzzling over it.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>