Fix Action Mailer changelog indenation to match other changelogs

Also some minor improvements to other changelogs. [ci skip]

activemodel/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
+
 *   Add `ActiveModel::Validations::AbsenceValidator`, a validator to check the
     absence of attributes.
 

activerecord/CHANGELOG.md

@@ -5,7 +5,7 @@
 
     *Yves Senn*
 
-*   Add ability for postgresql adapter to disable user triggers in disable_referential_integrity.
+*   Add ability for postgresql adapter to disable user triggers in `disable_referential_integrity`.
     Fix #5523
 
     *Gary S. Weaver*

activesupport/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
-*   Remove surrogate unicode character encoding from ActiveSupport::JSON.encode
+
+*   Remove surrogate unicode character encoding from `ActiveSupport::JSON.encode`
     The encoding scheme was broken for unicode characters outside the basic multilingual plane;
-    since json is assumed to be UTF-8, and we already force the encoding to UTF-8 simply pass through
-    the un-encoded characters.
+    since json is assumed to be `UTF-8`, and we already force the encoding to `UTF-8`,
+    simply pass through the un-encoded characters.
 
     *Brett Carter*