minor #18387 [Mailer] Document the optional HttpClient dependency (alexandre-daubois)

This PR was merged into the 5.4 branch.

Discussion
----------

[Mailer] Document the optional HttpClient dependency

Resolves #18386

Commits
-------

7ecb253 [Mailer] Document the optional HttpClient dependency

Author: javiereguiluz

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)