[Mailer] Document the optional HttpClient dependency

Author: alexandre-daubois

mailer.rst

@@ -215,6 +215,12 @@ Provider              SMTP                                                 HTTP
         The usage of ``default_socket_timeout`` as the default timeout was
         introduced in Symfony 5.1.
 
+.. note::
+
+    Besides SMTP, many 3rd party transports offer a web API to send emails.
+    To do so, you have to install (additionally to the bridge)
+    the HttpClient component via ``composer require symfony/http-client``.
+
 .. note::
 
     To use Google Gmail, you must have a Google Account with 2-Step-Verification (2FA)