minor #11054 Added caution-box about special characters (ThomasLandauer)

This PR was merged into the 4.2 branch.

Discussion
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Added caution-box about special characters

Copied caution-box about special characters from https://symfony.com/doc/current/doctrine.html#configuring-the-database

Commits
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52bc2b9 Update email.rst

Author: javiereguiluz

email.rst

@@ -39,11 +39,16 @@ environment variable in the ``.env`` file:
     # use this to disable email delivery
     MAILER_URL=null://localhost
 
-    # use this to configure a traditional SMTP server (make sure to URL-encode the
-    # values of the username and password if they contain non-alphanumeric characters
-    # such as '+', '@', ':' and '*', which are reserved in URLs)
+    # use this to configure a traditional SMTP server
     MAILER_URL=smtp://localhost:25?encryption=ssl&auth_mode=login&username=&password=
 
+.. caution::
+
+    If the username, password or host contain any character considered special in a
+    URI (such as ``+``, ``@``, ``$``, ``#``, ``/``, ``:``, ``*``, ``!``), you must
+    encode them. See `RFC 3986`_ for the full list of reserved characters or use the
+    :phpfunction:`urlencode` function to encode them.
+
 Refer to the :doc:`SwiftMailer configuration reference </reference/configuration/swiftmailer>`
 for the detailed explanation of all the available config options.