KVM: MMU: update the documentation for reverse mapping of parent_pte

Update the document to match the current reverse mapping of
parent_pte
Signed-off-by: NXiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Documentation/virtual/kvm/mmu.txt

@@ -191,12 +191,12 @@ Shadow pages contain the following information:
     A counter keeping track of how many hardware registers (guest cr3 or
     pdptrs) are now pointing at the page.  While this counter is nonzero, the
     page cannot be destroyed.  See role.invalid.
-  multimapped:
-    Whether there exist multiple sptes pointing at this page.
-  parent_pte/parent_ptes:
-    If multimapped is zero, parent_pte points at the single spte that points at
-    this page's spt.  Otherwise, parent_ptes points at a data structure
-    with a list of parent_ptes.
+  parent_ptes:
+    The reverse mapping for the pte/ptes pointing at this page's spt. If
+    parent_ptes bit 0 is zero, only one spte points at this pages and
+    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_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