Update README to better explain owner reference

The owner reference is available by default as `owner`, but the readme
did not make this clear.

README.md

@@ -132,32 +132,52 @@ The enum field works like any other model attribute. It can be mass-assigned usi
 
 ## Back reference to owning object
 
-In some cases you may want an enum class to be able to reference the owning object (an instance of the active record model). Think of it as a `belongs_to` relationship, where the enum can reference its owning object.
+In some cases you may want an enum class to reference the owning object
+(an instance of the active record model). Think of it as a `belongs_to`
+relationship, where the enum can reference its owning object.
 
-In order to create the back reference, you must declare how you wish to refer to the owner using the `owner` class method.
+By default, the back reference can be called using `owner`.
+If you want to refer to the owner by a different name, you must explicitly declare
+the owner name in the classy_enum parent class using the `owner` class method.
 
-For example:
+Example using the default `owner` method:
 
 ```ruby
 class Priority < ClassyEnum::Base
   enum_classes :low, :medium, :high
-  owner :alarm
 end
 
-class PriorityLow < Priority
+...
+# low and medium subclasses omitted
+...
+
+class PriorityHigh < Priority
+  def send_email?
+    owner.enabled?
+  end
 end
+```
 
-class PriorityMedium < Priority
+Example where the owner reference is explicitly declared:
+
+```ruby
+class Priority < ClassyEnum::Base
+  enum_classes :low, :medium, :high
+  owner :alarm
 end
 
+...
+# low and medium subclasses omitted
+...
+
 class PriorityHigh < Priority
   def send_email?
     alarm.enabled?
   end
 end
 ```
 
-In the above example, high priority alarms are only emailed if the owning alarm is enabled.
+In the above examples, high priority alarms are only emailed if the owning alarm is enabled.
 
 ```ruby
 @alarm = Alarm.create(:priority => :high, :enabled => true)