Add lambdas in conditional validations

As `Lambdas` are a type of `Proc`, they can also be used in the
`if`/`unless` option of a validation to decide when the validation is
executed. Add this case to the guide for clarification.

Closes https://github.com/rails/rails/issues/33212

guides/source/active_record_validations.md

@@ -927,6 +927,13 @@ class Account < ApplicationRecord
 end
 ```
 
+As `Lambdas` are a type of `Proc`, they can also be used to write inline
+conditions in a shorter way.
+
+```ruby
+validates :password, confirmation: true, unless: -> { password.blank? }
+```
+
 ### Grouping Conditional validations
 
 Sometimes it is useful to have multiple validations use one condition. It can