Rewrite the comment in FNAME(fetch) to explain why KVM needs to check that
the gPTE is still fresh before continuing the shadow page walk, even if
KVM already has a linked shadow page for the gPTE in question.

No functional change intended.

Link: https://lore.kernel.org/r/20240802203900.348808-4-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>