Clarify possible validations you can use to avoid a NULL boolean value

in the database. Closes #16304 [ci skip]

guides/source/active_record_validations.md

@@ -524,9 +524,15 @@ If you validate the presence of an object associated via a `has_one` or
 `marked_for_destruction?`.
 
 Since `false.blank?` is true, if you want to validate the presence of a boolean
-field you should use `validates :field_name, inclusion: { in: [true, false] }`.
+field you should use one of the following validations:
 
-The default error message is _"can't be blank"_.
+```ruby
+validates :boolean_field_name, presence: true
+validates :boolean_field_name, inclusion: { in: [true, false] }
+validates :boolean_field_name, exclusion: { in: [nil] }
+```
+By using one of these validations, you will ensure the value will NOT be `nil`
+which would result in a `NULL` value in most cases.
 
 ### `absence`