Add associated objects display methods to docs

docs/3-index-pages/index-as-table.md

@@ -15,6 +15,13 @@ column method:
       column :title
     end
 
+For association columns we make an educated guess on what to display by
+calling the following methods in the following order:
+
+    :display_name, :full_name, :name, :username, :login, :title, :email, :to_s
+
+This can be customized in config/initializers/active_admin.rb.
+
 If the default title does not work for you, pass it as the first argument:
 
     index do
@@ -53,7 +60,6 @@ Alternatively, you can create a column with custom links:
       end
     end
 
-
 ## Sorting
 
 When a column is generated from an Active Record attribute, the table is
@@ -87,4 +93,4 @@ For example, if you were using CanCan:
       if can? :manage, Post
         column :some_secret_data
       end
-    end
+    end