Added a tip about HttpOptions::setHeaders vs HttpOptions::addHeader

Fixes symfony#19236

Not sure of the documentation format. Open to suggestion to improve it :)

Author: Dean151

http_client.rst

@@ -154,6 +154,15 @@ brings most of the available options with type-hinted getters and setters::
             ->toArray()
     );
 
+.. tip::
+
+    Be aware that calling :method:`Symfony\\Component\\HttpClient\\HttpOptions::setHeaders` overrides all previous headers.
+    If you intend to add or replace only one header, use :method:`Symfony\\Component\\HttpClient\\HttpOptions::addHeader` instead.
+
+.. versionadded:: 7.1
+
+    The :method:`Symfony\\Component\\HttpClient\\HttpOptions::addHeader` method was introduced in Symfony 7.1.
+
 Some options are described in this guide:
 
 * `Authentication`_