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

    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