Update xml/System.Runtime.CompilerServices/ExtensionMarkerAttribute.xml

Author: ericstj

xml/System.Runtime.CompilerServices/ExtensionMarkerAttribute.xml

@@ -30,7 +30,7 @@
   <Docs>
     <summary>
       <para>Represents an attribute that's used to mark extension members and associate them with a specific marker type (which provides detailed information about an extension block and its receiver parameter).</para>
-      <para>This attribute should not be used by developers in source code.</para>
+      <forInternalUseOnly />
     </summary>
     <remarks>To be added.</remarks>
   </Docs>