Add proxy credentials rematk to HttpHandler.UseDefaultCredentials

Author: Jan Jahoda

xml/System.Net.Http/HttpClientHandler.xml

@@ -1087,7 +1087,8 @@ handler.ServerCertificateCustomValidationCallback = HttpClientHandler.DangerousA
 
 ## Remarks
  Set this property to `true` when requests made by the <xref:System.Net.Http.HttpClientHandler> object should, if requested by the server, be authenticated using the credentials of the currently logged on user. For client applications, this is the desired behavior in most scenarios. For middle-tier applications, such as ASP.NET applications, instead of using this property, you would typically set the <xref:System.Net.Http.HttpClientHandler.Credentials%2A> property to the credentials of the client on whose behalf the request is made.
-
+ 
+ This property doesn't affect proxy credentials. Proxy credentials need to be explicitly set by the <xref:System.Net.Http.HttpClientHandler.DefaultProxyCredentials> property. 
  ]]></format>
         </remarks>
       </Docs>