x86/apic, doc: Justification for disabling IO APIC before Local APIC

Since erratum AVR31 in "Intel Atom Processor C2000 Product Family
Specification Update" is now published, I added a justification
comment for disabling IO APIC before Local APIC, as changed in commit:

522e6646

 x86/apic: Disable I/O APIC before shutdown of the local APIC

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1386202069-51515-1-git-send-email-fenghua.yu@intel.com


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>