Add NotSupportedException on IEnumerator.Reset

Author: carlreinke

xml/System.Collections/IEnumerator.xml

@@ -285,6 +285,7 @@
         <altmember cref="M:System.Collections.IEnumerator.MoveNext" />
         <altmember cref="P:System.Collections.IEnumerator.Current" />
         <exception cref="T:System.InvalidOperationException">The collection was modified after the enumerator was created. </exception>
+        <exception cref="T:System.NotSupportedException">The enumerator does not support being reset.</exception>
       </Docs>
     </Member>
   </Members>