KVM: x86/mmu: add comment explaining arguments to kvm_zap_gfn_range

This comment makes it clear that the range of gfns that this
function receives is non inclusive.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210810205251.424103-4-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>