Improve changelogs

Also make Action Mailer changelog format more consistent with the
others [ci skip]

actionmailer/CHANGELOG.md

@@ -1,12 +1,14 @@
-* Instrument the generation of Action Mailer messages. The time it takes to
-  generate a message is written to the log.
+*   Instrument the generation of Action Mailer messages. The time it takes to
+    generate a message is written to the log.
 
-  *Daniel Schierbeck*
+    *Daniel Schierbeck*
 
-* invoke mailer defaults as procs only if they are procs, do not convert
-  with to_proc.  That an object is convertible to a proc does not mean it's
-  meant to be always used as a proc.  Fixes #11533
+*   Invoke mailer defaults as procs only if they are procs, do not convert with
+    `to_proc`. That an object is convertible to a proc does not mean it's meant
+    to be always used as a proc.
 
-  *Alex Tsukernik*
+    Fixes #11533.
+
+    *Alex Tsukernik*
 
 Please check [4-0-stable](https://github.com/rails/rails/blob/4-0-stable/actionmailer/CHANGELOG.md) for previous changes.

actionpack/CHANGELOG.md

@@ -2,7 +2,7 @@
 
     *Sıtkı Bağdat*
 
-*   Take a hash with options inside array in #url_for.
+*   Take a hash with options inside array in `#url_for`.
 
     Example:
 

activerecord/CHANGELOG.md

@@ -1,5 +1,4 @@
-*   Make ActiveRecord::Relation#unscope affect relations it is merged in
-    to.
+*   Make `ActiveRecord::Relation#unscope` affect relations it is merged in to.
 
     *Jon Leighton*
 

railties/CHANGELOG.md

@@ -1,6 +1,7 @@
 *   Remove turbolinks when generating a new application based on a template that skips it.
 
-    Example
+    Example:
+
         Skips turbolinks adding `add_gem_entry_filter { |gem| gem.name != "turbolinks" }`
         to the template.