Document System.MidpointRounding enum (#2853)

* Document System.MidpointRounding enum

* suggestion by tannergooding

Author: carlossanlop

xml/System/MidpointRounding.xml

@@ -187,7 +187,7 @@
       </ReturnValue>
       <MemberValue>3</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>When a number is halfway between two others, it is rounded toward the result closest to and no greater than the infinitely precise result.</summary>
       </Docs>
     </Member>
     <Member MemberName="ToPositiveInfinity">
@@ -219,7 +219,7 @@
       </ReturnValue>
       <MemberValue>4</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>When a number is halfway between two others, it is rounded toward the result closest to and no less than the infinitely precise result.</summary>
       </Docs>
     </Member>
     <Member MemberName="ToZero">
@@ -251,8 +251,8 @@
       </ReturnValue>
       <MemberValue>2</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>When a number is halfway between two others, it is rounded toward the result closest to and no greater in magnitude than the infinitely precise result.</summary>
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>