Remind users that the User Agent may be null. (dotnet#725)

When you read the page, it appears that a String will be returned always, this is not the case. In the event the web requester sends a null User Agent, it is not converted to an empty string, and thus should be considered by the programmer.

Author: vthg2themax

xml/System.Web/HttpRequest.xml

@@ -2251,7 +2251,7 @@ Dim fullname2 As String = Request("fullname")
         <ReturnType>System.String</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Gets the raw user agent string of the client browser.</summary>
+        <summary>Gets the raw user agent string of the client browser that has been provided. Please note it may be null.</summary>
         <value>The raw user agent string of the client browser.</value>
         <remarks>
           <format type="text/markdown"><![CDATA[  
@@ -2408,4 +2408,4 @@ Dim fullname2 As String = Request("fullname")
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>