Added docs for new MessageBox Button and Result APIs (#11340)

Author: dipeshmsft

xml/System.Windows/MessageBoxButton.xml

@@ -61,7 +61,7 @@ The value of the button that a user clicks is returned by <xref:System.Windows.M
       </ReturnValue>
       <MemberValue>2</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>The message box displays **Abort**, **Retry**, and **Ignore** buttons.</summary>
       </Docs>
     </Member>
     <Member MemberName="CancelTryContinue">
@@ -81,7 +81,7 @@ The value of the button that a user clicks is returned by <xref:System.Windows.M
       </ReturnValue>
       <MemberValue>6</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>The message box displays **Cancel**, **Retry**, and **Continue** buttons.</summary>
       </Docs>
     </Member>
     <Member MemberName="OK">
@@ -155,7 +155,7 @@ The value of the button that a user clicks is returned by <xref:System.Windows.M
       </ReturnValue>
       <MemberValue>5</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>The message box displays **Retry and **Cancel** buttons.</summary>
       </Docs>
     </Member>
     <Member MemberName="YesNo">

xml/System.Windows/MessageBoxResult.xml

@@ -67,7 +67,7 @@
       </ReturnValue>
       <MemberValue>3</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>The result value of the message box is **Abort** (usually sent from a button labeled Abort).</summary>
       </Docs>
     </Member>
     <Member MemberName="Cancel">
@@ -114,7 +114,7 @@
       </ReturnValue>
       <MemberValue>11</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>The result value of the message box is **Continue** (usually sent from a button labeled Continue).</summary>
       </Docs>
     </Member>
     <Member MemberName="Ignore">
@@ -134,7 +134,7 @@
       </ReturnValue>
       <MemberValue>5</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>The result value of the message box is **Ignore** (usually sent from a button labeled Ignore).</summary>
       </Docs>
     </Member>
     <Member MemberName="No">
@@ -235,7 +235,7 @@
       </ReturnValue>
       <MemberValue>4</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>The result value of the message box is **Retry** (usually sent from a button labeled Retry).</summary>
       </Docs>
     </Member>
     <Member MemberName="TryAgain">
@@ -255,7 +255,7 @@
       </ReturnValue>
       <MemberValue>10</MemberValue>
       <Docs>
-        <summary>To be added.</summary>
+        <summary>The result value of the message box is **TryAgain** (usually sent from a button labeled Try Again).</summary>
       </Docs>
     </Member>
     <Member MemberName="Yes">