Update description for UploadString() method. (#4015)

The remarks section uses the following text for both the `UploadString()` and `UploadStringAsync()` methods:

> Sends a String to the resource, without blocking the calling thread.

I think this is a copy-and-paste error because only the `UploadStringAsync()` method is non-blocking. I have updated the description for `UploadString()` to follow the pattern used when describing other upload methods.

Author: ericmutta

xml/System.Net/WebClient.xml

@@ -72,7 +72,7 @@
 |<xref:System.Net.WebClient.UploadFileAsync%2A>|Sends a local file to the resource, without blocking the calling thread.|  
 |<xref:System.Net.WebClient.UploadValues%2A>|Sends a <xref:System.Collections.Specialized.NameValueCollection> to the resource and returns a <xref:System.Byte> array containing any response.|  
 |<xref:System.Net.WebClient.UploadValuesAsync%2A>|Sends a <xref:System.Collections.Specialized.NameValueCollection> to the resource and returns a <xref:System.Byte> array containing any response, without blocking the calling thread.|  
-|<xref:System.Net.WebClient.UploadString%2A>|Sends a <xref:System.String> to the resource, without blocking the calling thread.|  
+|<xref:System.Net.WebClient.UploadString%2A>|Sends a <xref:System.String> to the resource and returns a <xref:System.String> containing any response.|  
 |<xref:System.Net.WebClient.UploadStringAsync%2A>|Sends a <xref:System.String> to the resource, without blocking the calling thread.|  
 
  The following table describes <xref:System.Net.WebClient> methods for downloading data from a resource.