KVM: x86/mmu: Consolidate comments about {Host,MMU}-writable

Consolidate the large comment above DEFAULT_SPTE_HOST_WRITABLE with the
large comment above is_writable_pte() into one comment. This comment
explains the different reasons why an SPTE may be non-writable and KVM
keeps track of that with the {Host,MMU}-writable bits.

No functional change intended.

Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20220125230723.1701061-1-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>