x86/Kconfig: Correct X86_X2APIC help text

Currently, it is not true that the kernel will panic with CONFIG_X86_X2APIC=n
on systems that require it; it will try to disable the APIC and run without
it to at least give the user a clear warning message. See the second
variant of check_x2apic() in arch/x86/kernel/apic/apic.c .

Also massage some other parts of the help text.

Fixes: 9232c49f ("x86/Kconfig: Enable X86_X2APIC by default and improve help text")
Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250322154541.40325-1-mat.jonczyk@o2.pl