enhance property description (#2385)

Author: mairaw

xml/System.Threading.Tasks.Dataflow/DataflowBlockOptions.xml

@@ -143,8 +143,9 @@
         <ReturnType>System.Boolean</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Gets or sets whether ordered processing should be enforced on a block's handling of messages.</summary>
-        <value>To be added.</value>
+        <summary>Gets or sets a value that indicates whether ordered processing should be enforced on a block's handling of messages.</summary>
+        <value>
+          <see langword="true" /> if ordered processing should be enforced; otherwise, <see langword="false" />. The default value is <see langword="true" />.</value>
         <remarks>
           <format type="text/markdown"><![CDATA[