Document System.IO.MatchCasing (#3528)

Author: carlossanlop

xml/System.IO/MatchCasing.xml

@@ -17,7 +17,7 @@
     <BaseTypeName>System.Enum</BaseTypeName>
   </Base>
   <Docs>
-    <summary>To be added.</summary>
+    <summary>Specifies the type of character casing to match.</summary>
     <remarks>To be added.</remarks>
   </Docs>
   <Members>