Add code examples to `readonly` documentation

activerecord/lib/active_record/core.rb

@@ -697,6 +697,10 @@ module ActiveRecord
     end
 
     # Marks this record as read only.
+    #
+    #   customer = Customer.first
+    #   customer.readonly!
+    #   customer.save # Raises an ActiveRecord::ReadOnlyRecord
     def readonly!
       @readonly = true
     end

guides/source/active_record_querying.md

@@ -1305,7 +1305,7 @@ Active Record provides the [`readonly`][] method on a relation to explicitly dis
 ```ruby
 customer = Customer.readonly.first
 customer.visits += 1
-customer.save
+customer.save # Raises an ActiveRecord::ReadOnlyRecord
 ```
 
 As `customer` is explicitly set to be a readonly object, the above code will raise an `ActiveRecord::ReadOnlyRecord` exception when calling `customer.save` with an updated value of _visits_.