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 <[e-mail]>
Reviewed-by: Kai Huang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>

Documentation/virt/kvm/x86/mmu.rst

···
     since the last time the page table was actually used; if emulation
     is triggered too frequently on this page, KVM will unmap the page
     to avoid emulation in the future.
+  tdp_mmu_page:
+    Is 1 if the shadow page is a TDP MMU page. This variable is used to
+    bifurcate the control flows for KVM when walking any data structure that
+    may contain pages from both TDP MMU and shadow MMU.
 
 Reverse map
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