x86/nmi: Improve and relocate NMI handler comments

x86/nmi: Improve and relocate NMI handler comments

Some of the comments in the default NMI handling code are out of place
or inadequate. Move them to the appropriate locations and update them as
needed.

Move the comment related to CPU-specific NMIs closer to the actual code.
Also, add more details about how back-to-back NMIs are detected since
that isn't immediately obvious.

Opportunistically, replace an #ifdef section in the vicinity with an
IS_ENABLED() check to make the code easier to read.

Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Kai Huang <kai.huang@intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Link: https://lore.kernel.org/r/20250327234629.3953536-7-sohil.mehta@intel.com