x86: update reference for PAE tlb flushing

Remove bogus reference to "Pentium-II erratum A13" and point to the
actual canonical source of information about what requirements x86
processors have for PAE pagetable updates.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>