Docs: Update to_xml documentation to match as_json docuemntation

activemodel/lib/active_model/serializers/xml.rb

@@ -134,6 +134,31 @@ def add_procs
 
       # Returns XML representing the model. Configuration can be
       # passed through +options+.
+      #
+      # Without any +options+, the returned XML string will include all the model's
+      # attributes. For example:
+      #
+      #   konata = User.find(1)
+      #   konata.to_xml
+      #
+      #   <?xml version="1.0" encoding="UTF-8"?>
+      #   <user>
+      #     <id type="integer">1</id>
+      #     <name>David</name>
+      #     <age type="integer">16</age>
+      #     <created-at type="datetime">2011-01-30T22:29:23Z</created-at>  
+      #   </user>
+      #
+      # The <tt>:only</tt> and <tt>:except</tt> options can be used to limit the attributes
+      # included, and work similar to the +attributes+ method.
+      # 
+      # To include the result of some method calls on the model use <tt>:methods</tt>
+      #
+      # To include associations use <tt>:include</tt>
+      #
+      # For further documentation see activerecord/lib/active_record/serializers/xml_serializer.xml
+      
+      
       def to_xml(options = {}, &block)
         Serializer.new(self, options).serialize(&block)
       end