KVM: Documentation: Add the missing description for ptep in kvm_mmu_page

Add the missing description for ptep in kvm_mmu_page description. ptep is
used when TDP MMU is enabled and it shares the storage with parent_ptes.
Update the doc to help readers to get up-to-date info.
Signed-off-by: Mingwei Zhang <mizhang@google.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20230912184553.1887764-4-mizhang@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

Documentation/virt/kvm/x86/mmu.rst

@@ -239,6 +239,9 @@ Shadow pages contain the following information:
     parent_ptes points at this single spte, otherwise, there exists multiple
     sptes pointing at this page and (parent_ptes & ~0x1) points at a data
     structure with a list of parent sptes.
+  ptep:
+    The kernel virtual address of the SPTE that points at this shadow page.
+    Used exclusively by the TDP MMU, this field is a union with parent_ptes.
   unsync:
     If true, then the translations in this page may not match the guest's
     translation.  This is equivalent to the state of the tlb when a pte is