documents how to DROP DEFAULT [ci skip]

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -359,11 +359,15 @@ def change_column(table_name, column_name, type, options = {})
         raise NotImplementedError, "change_column is not implemented"
       end
 
-      # Sets a new default value for a column.
+      # Sets a new default value for a column:
+      #
+      #   change_column_default(:suppliers, :qualification, 'new')
+      #   change_column_default(:accounts, :authorized, 1)
+      #
+      # Setting the default to +nil+ effectively drops the default:
       #
-      #  change_column_default(:suppliers, :qualification, 'new')
-      #  change_column_default(:accounts, :authorized, 1)
       #  change_column_default(:users, :email, nil)
+      #
       def change_column_default(table_name, column_name, default)
         raise NotImplementedError, "change_column_default is not implemented"
       end