Merge branch '5.4' into 6.2

* 5.4:
  [Mailer] Document the optional HttpClient dependency

Author: javiereguiluz

mailer.rst

@@ -197,6 +197,12 @@ Provider              SMTP                                                 HTTP
     When using SMTP, the default timeout for sending a message before throwing an
     exception is the value defined in the `default_socket_timeout`_ PHP.ini option.
 
+.. 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)