KVM: nVMX: Reword comments about generating nested CR0/4 read shadows

Reword the comments that (attempt to) document nVMX's overrides of the
CR0/4 read shadows for L2 after calling vmx_set_cr0/4().  The important
behavior that needs to be documented is that KVM needs to override the
shadows to account for L1's masks even though the shadows are set by the
common helpers (and that setting the shadows first would result in the
correct shadows being clobbered).

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Link: https://lore.kernel.org/r/20220831000721.4066617-1-seanjc@google.com