x86/nmi: Improve <asm/nmi.h> documentation

x86/nmi: Improve <asm/nmi.h> documentation

NMI handlers can be registered by various subsystems, including drivers.

However, the interface for registering and unregistering such handlers
is not clearly documented. In the future, the interface may need to be
extended to identify the source of the NMI.

Add documentation to make the current API more understandable and easier
to use.

Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Link: https://lore.kernel.org/r/20250327234629.3953536-8-sohil.mehta@intel.com