KVM: x86/mmu: Document the "rules" for using host_pfn_mapping_level()

Add a comment to document how host_pfn_mapping_level() can be used safely,
as the line between safe and dangerous is quite thin.  E.g. if KVM were
to ever support in-place promotion to create huge pages, consuming the
level is safe if the caller holds mmu_lock and checks that there's an
existing _leaf_ SPTE, but unsafe if the caller only checks that there's a
non-leaf SPTE.

Opportunistically tweak the existing comments to explicitly document why
KVM needs to use READ_ONCE().

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220715232107.3775620-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>