KVM: Documentation: Add the missing description for guest_mode in kvm_mmu_page_role

Add the missing description for guest_mode in kvm_mmu_page_role
description.  guest_mode tells KVM whether a shadow page is used for the L1
or an L2. Update the missing field in documentation.

Signed-off-by: Mingwei Zhang <mizhang@google.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20230912184553.1887764-2-mizhang@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: mzhang3579

Documentation/virt/kvm/x86/mmu.rst

@@ -202,6 +202,8 @@ Shadow pages contain the following information:
     Is 1 if the MMU instance cannot use A/D bits.  EPT did not have A/D
     bits before Haswell; shadow EPT page tables also cannot use A/D bits
     if the L1 hypervisor does not enable them.
+  role.guest_mode:
+    Indicates the shadow page is created for a nested guest.
   role.passthrough:
     The page is not backed by a guest page table, but its first entry
     points to one.  This is set if NPT uses 5-level page tables (host