powerpc: Document how we set AIL on guest kernels

I spent ten minutes scratching my head, trying to work out where we
enabled relocation on interrupts for guest kernels. Expand the doco to
make it clear.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>