Documentation: admin-guide: Document side effects when pKVM is enabled

Recent changes to KVM for arm64 has made it impossible for the host to
hibernate or use kexec when protected mode is enabled via the kernel
command line.

There are people who rely on kexec (for example, developers who use kexec
as a quick way to test a new kernel), let's document this change in
behaviour, so it doesn't catch them by surprise and we have a place to
point people to if it does.
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211011153835.291147-1-alexandru.elisei@arm.com

Documentation/admin-guide/kernel-parameters.txt

@@ -2372,7 +2372,9 @@
 				   state is kept private from the host.
 				   Not valid if the kernel is running in EL2.
 
-			Defaults to VHE/nVHE based on hardware support.
+			Defaults to VHE/nVHE based on hardware support. Setting
+			mode to "protected" will disable kexec and hibernation
+			for the host.
 
 	kvm-arm.vgic_v3_group0_trap=
 			[KVM,ARM] Trap guest accesses to GICv3 group-0