KVM: MMU: document write_flooding_count

Document write_flooding_count 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 5d28c11..6b636fd 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -233,6 +233,7 @@
 	int clear_spte_count;
 #endif
 
+	/* Number of writes since the last time traversal visited this page.  */
 	int write_flooding_count;
 };