powerpc: Document irq enable/disable toggle in migrate_irqs()

[Michael Ellerman]

On Wed, 2017-02-15 at 09:49:54 UTC, Michael Ellerman wrote:
> This code is undocumented and unclear, and we've already had one patch
> sent to remove it because it's "paradoxical and unnecessary". So
> document it to save our future selves from puzzling over it.
> 
> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>

Applied to powerpc next.

https://git.kernel.org/powerpc/c/687b8f24f14db842c8c3f8cb8b24c9

cheers