Merge pull request #20215 from repinel/validates_acceptance_doc

[ci skip] add to doc that acceptance validator allows `nil` by default

guides/source/active_record_validations.md

@@ -283,6 +283,7 @@ class Person < ActiveRecord::Base
 end
 ```
 
+This check is performed only if `terms_of_service` is not `nil`.
 The default error message for this helper is _"must be accepted"_.
 
 It can receive an `:accept` option, which determines the value that will be