docs, add update_all example with SQL fragment. [ci skip]

The relation method `update_all` allows you to pass a SQL fragment. The
functionality is already mentioned in the prose but the examples section
does not cover it.

activerecord/lib/active_record/relation.rb

@@ -372,6 +372,9 @@ def scoping
     #
     #   # Update all books that match conditions, but limit it to 5 ordered by date
     #   Book.where('title LIKE ?', '%Rails%').order(:created_at).limit(5).update_all(author: 'David')
+    #
+    #   # Update all invoices and set the number column to its id value.
+    #   Invoice.update_all('number = id')
     def update_all(updates)
       raise ArgumentError, "Empty list of attributes to change" if updates.blank?