KVM: MMU: document clear_spte_count

Document it to Documentation/virtual/kvm/mmu.txt

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 966f265..5d28c11 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -226,6 +226,10 @@
 	DECLARE_BITMAP(unsync_child_bitmap, 512);
 
 #ifdef CONFIG_X86_32
+	/*
+	 * Used out of the mmu-lock to avoid reading spte values while an
+	 * update is in progress; see the comments in __get_spte_lockless().
+	 */
 	int clear_spte_count;
 #endif