Update the documentation for find_by method

Author: bodhish

guides/source/active_record_querying.md

@@ -358,6 +358,8 @@ The SQL equivalent of the above is:
 SELECT * FROM customers WHERE (customers.first_name = 'Lifo') LIMIT 1
 ```
 
+Note that there is no `ORDER BY` in the above SQL.  If your `find_by` conditions can match multiple records, you should [apply an order](#ordering) to guarantee a deterministic result.
+
 The [`find_by!`][] method behaves exactly like `find_by`, except that it will raise `ActiveRecord::RecordNotFound` if no matching record is found. For example:
 
 ```irb