KVM: document 'kvm.mmu_audit' parameter

Document this parameter into Documentation/kernel-parameters.txt
Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

Documentation/kernel-parameters.txt

@@ -1126,6 +1126,10 @@ and is between 256 and 4096 characters. It is defined in the file
 	kvm.oos_shadow=	[KVM] Disable out-of-sync shadow paging.
 			Default is 1 (enabled)
 
+	kvm.mmu_audit=	[KVM] This is a R/W parameter which allows audit
+			KVM MMU at runtime.
+			Default is 0 (off)
+
 	kvm-amd.nested=	[KVM,AMD] Allow nested virtualization in KVM/SVM.
 			Default is 1 (enabled)