Correct CngExportProperties.None enum description

Author: joynoele

xml/System.Security.Cryptography/CngExportPolicies.xml

@@ -259,7 +259,7 @@
       </ReturnValue>
       <MemberValue>0</MemberValue>
       <Docs>
-        <summary>No export policies are established. Key export is allowed without restriction.</summary>
+        <summary>The key is not exportable.</summary>
       </Docs>
     </Member>
   </Members>