diff --git a/activerecord/lib/active_record/persistence.rb b/activerecord/lib/active_record/persistence.rb
index b587abd5d0..869cbf5b2e 100644
--- a/activerecord/lib/active_record/persistence.rb
+++ b/activerecord/lib/active_record/persistence.rb
@@ -110,6 +110,10 @@ module ActiveRecord
     # * updated_at/updated_on column is updated if that column is available
     # * does not work on associations
     # * does not work on attr_accessor attributes. The attribute that is being updated must be column name.
+    # * does not work on new record. <tt>record.new_record?</tt> should return false for this method to work.
+    # * updates only the attribute that is input to the method. If there are other changed attributes then
+    #   those attributes are left alone. In that case even after this method has done its work <tt>record.changed?</tt>
+    #   will return true.
     #
     def update_attribute(name, value)
       raise ActiveRecordError, "#{name.to_s} is marked as readonly" if self.class.readonly_attributes.include? name.to_s