KVM: Documentation: Add the missing description for tdp_mmu_page into kvm_mmu_page

Add the description for tdp_mmu_page into kvm_mmu_page description.
tdp_mmu_page is a field to differentiate shadow pages from TDP MMU and
non-TDP MMU.

Signed-off-by: Mingwei Zhang <mizhang@google.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20230912184553.1887764-7-mizhang@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>