Documentation: Document arm64 kpti control

For a while Arm64 has been capable of force enabling
or disabling the kpti mitigations. Lets make sure the
documentation reflects that.
Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/kernel-parameters.txt

@@ -1983,6 +1983,12 @@
 			Built with CONFIG_DEBUG_KMEMLEAK_DEFAULT_OFF=y,
 			the default is off.
 
+	kpti=		[ARM64] Control page table isolation of user
+			and kernel address spaces.
+			Default: enabled on cores which need mitigation.
+			0: force disabled
+			1: force enabled
+
 	kvm.ignore_msrs=[KVM] Ignore guest accesses to unhandled MSRs.
 			Default is 0 (don't ignore, but inject #GP)