minor #1116 add values to parameters copied from symfony docs (pscheit)

This PR was submitted for the master branch but it was merged into the 1.2 branch instead (closes #1116).

Discussion
----------

add values to parameters copied from symfony docs

Commits
-------

68492a0e add values to parameters copied from symfony docs

doc/providers/swiftmailer.rst

@@ -21,8 +21,8 @@ Parameters
   * **port**: SMTP port, defaults to 25.
   * **username**: SMTP username, defaults to an empty string.
   * **password**: SMTP password, defaults to an empty string.
-  * **encryption**: SMTP encryption, defaults to null.
-  * **auth_mode**: SMTP authentication mode, defaults to null.
+  * **encryption**: SMTP encryption, defaults to null. Valid values are 'tls', 'ssl', or null (indicating no encryption).
+  * **auth_mode**: SMTP authentication mode, defaults to null. Valid values are 'plain', 'login', 'cram-md5', or null.
 
   Example usage::