recover dropout content (#4152)

Author: mairaw

xml/ns-System.Xml.xml

@@ -32,7 +32,7 @@
   
 <a name="async"></a>   
 ## Processing XML asynchronously  
- The <xref:System.Xml.XmlReader?displayProperty=nameWithType> and <xref:System.Xml.XmlWriter?displayProperty=nameWithType> classes include a number of asynchronous methods that are based on the . These methods can be identified by the string "Async" at the end of their names. With these methods, you can write asynchronous code that's similar to your synchronous code, and you can migrate your existing synchronous code to asynchronous code easily.  
+ The <xref:System.Xml.XmlReader?displayProperty=nameWithType> and <xref:System.Xml.XmlWriter?displayProperty=nameWithType> classes include a number of asynchronous methods that are based on the asynchronous programming model. These methods can be identified by the string "Async" at the end of their names. With these methods, you can write asynchronous code that's similar to your synchronous code, and you can migrate your existing synchronous code to asynchronous code easily.  
   
 -   Use the asynchronous methods in apps where there is significant network stream latency. Avoid using the asynchronous APIs for memory stream or local file stream read/write operations. The input stream, <xref:System.Xml.XmlTextReader>, and <xref:System.Xml.XmlTextWriter> should support asynchronous operations as well. Otherwise, threads will still be blocked by I/O operations.  
   
@@ -245,4 +245,4 @@
     <altmember cref="N:System.Xml.Linq" />
     <related type="Article" href="~/docs/standard/data/xml/index.md">XML Documents and Data</related>
   </Docs>
-</Namespace>
+</Namespace>