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