x86: nmi - nmi_watchdog boot param docs cleanup

Impact: documentation update

1) nmi_watchdog boot parameter is common to 32/64 bit modes. So
   move it from Documentation/x86/x86_64/boot-options.txt to
   Documentation/kernel-parameters.txt and integrate with.

2) Also fix [panic] keyword placement -- it ought to be at first
   position otherwise it will not be recognized.

3) Document lapic and ioapic keywords.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>