Added documentation for DataflowBlockOptions.EnsureOrdered (#2374)

* Added documentation for DataflowBlockOptions.EnsureOrdered

* Apply suggestions from code review

Co-Authored-By: svick <[email protected]>

Author: svick

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

@@ -143,9 +143,23 @@
         <ReturnType>System.Boolean</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>To be added.</summary>
-        <value>To be added.</value>
-        <remarks>To be added.</remarks>
+        <summary>Gets or sets whether ordered processing should be enforced on a block's handling of messages.</summary>
+        <remarks>
+          <format type="text/markdown"><![CDATA[
+
+## Remarks
+
+By default, dataflow blocks enforce ordering on the processing of messages. This means that a
+block like <xref:System.Threading.Tasks.Dataflow.TransformBlock%602?displayProperty=nameWithType> will ensure that messages are output in the same
+order they were input, even if parallelism is employed by the block, and the processing of a message N finishes 
+after the processing of a subsequent message N+1 (the block will reorder the results to maintain the input
+ordering prior to making those results available to a consumer).  Some blocks may allow this to be relaxed,
+however.  Setting <xref:System.Threading.Tasks.Dataflow.DataflowBlockOptions.EnsureOrdered> to false tells a block that it may relax this ordering if
+it's able to do so.  This can be beneficial if making a processed result immediately available
+is more important than maintaining the input-to-output ordering.
+
+]]></format>
+        </remarks>
       </Docs>
     </Member>
     <Member MemberName="MaxMessagesPerTask">
@@ -260,4 +274,4 @@
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>