Document possibility to use user:pass for basic auth

Author: Maximilian Ruta

http_client.rst

@@ -487,6 +487,11 @@ each request (which overrides any global authentication):
         // ...
     ]);
 
+.. note::
+
+    Basic Authentication can be set by authority in the URL, like
+    http://user:pass@example.com/.
+
 .. note::
 
     The NTLM authentication mechanism requires using the cURL transport.