promote :except option instead of :only for before action docs [ci skip]

guides/source/security.md

@@ -572,7 +572,7 @@ NOTE: _When sanitizing, protecting or verifying something, prefer whitelists ove
 
 A blacklist can be a list of bad e-mail addresses, non-public actions or bad HTML tags. This is opposed to a whitelist which lists the good e-mail addresses, public actions, good HTML tags and so on. Although sometimes it is not possible to create a whitelist (in a SPAM filter, for example), _prefer to use whitelist approaches_:
 
-* Use before_action only: [...] instead of except: [...]. This way you don't forget to turn it off for newly added actions.
+* Use before_action except: [...] instead of only: [...] for security-related actions. This way you don't forget to enable security checks for newly added actions.
 * Allow <strong> instead of removing <script> against Cross-Site Scripting (XSS). See below for details.
 * Don't try to correct user input by blacklists:
     * This will make the attack work: "<sc<script>ript>".gsub("<script>", "")