[ActiveRecord] Bang version of named scopes

merge! and all the bang methods are private API and SHOULD NOT be used by the final users. This is why they are marked as nodoc.

For the same reason we changed these method to be private API, I think we are not interested on this feature.

See https://github.com/rails/rails/commit/c5bdf6c5aee61848bee67b307287e2f28ddca173