Update mailer.rst

Change for a better information see on https://symfony.com/doc/current/email.html

Author: cadjou

mailer.rst

@@ -67,7 +67,10 @@ over SMTP by configuring the DSN in your ``.env`` file (the ``user``,
 
 .. caution::
 
-    If your user or password has special characters such as @ or #, use encoding characters such as% 40 or% 23.
+    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.
 
 .. caution::