Added headings to serializers.

activemodel/lib/active_model/locale/en.yml

 en:
   errors:
-    # The default format use in full error messages.
+    # The default format to use in full error messages.
     format: "%{attribute} %{message}"
 
     # The values :model, :attribute and :value are always available for interpolation

activemodel/lib/active_model/serializers/json.rb

@@ -2,6 +2,7 @@
 require 'active_support/core_ext/class/attribute'
 
 module ActiveModel
+  # == Active Model JSON Serializer
   module Serializers
     module JSON
       extend ActiveSupport::Concern
@@ -14,8 +15,8 @@ module JSON
         self.include_root_in_json = true
       end
 
-      # Returns a JSON string representing the model. Some configuration is
-      # available through +options+.
+      # Returns a JSON string representing the model. Some configuration can be
+      # passed through +options+.
       #
       # The option <tt>ActiveModel::Base.include_root_in_json</tt> controls the
       # top-level behavior of to_json. It is true by default. When it is <tt>true</tt>,

activemodel/lib/active_model/serializers/xml.rb

@@ -5,6 +5,7 @@
 require 'active_support/core_ext/hash/slice'
 
 module ActiveModel
+  # == Active Model XML Serializer
   module Serializers
     module Xml
       extend ActiveSupport::Concern
@@ -131,6 +132,8 @@ def add_procs
         end
       end
 
+      # Returns XML representing the model. Configuration can be
+      # passed through +options+.
       def to_xml(options = {}, &block)
         Serializer.new(self, options).serialize(&block)
       end