minor #20045 [HttpClient] Document usage or user:pass@example.com for Basic Auth (Maximilian Ruta)

This PR was submitted for the 7.1 branch but it was merged into the 5.4 branch instead.

Discussion
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[HttpClient] Document usage or user:pass@example.com for Basic Auth

It is possible to use `username:password` in the current implementation. This behavior should be documented.

Commits
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54fc8f9 Document possibility to use user:pass for basic auth

Author: javiereguiluz

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.