KVM: x86/mmu: Rename unsync helper and update related comments

Rename mmu_need_write_protect() to mmu_try_to_unsync_pages() and update
a variety of related, stale comments.  Add several new comments to call
out subtle details, e.g. that upper-level shadow pages are write-tracked,
and that can_unsync is false iff KVM is in the process of synchronizing
pages.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622175739.3610207-14-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>