Correct the `allow_nil` and `allow_blank` documentation in ActiveModel::Validations Validators

- remove allow_nil, allow_blank doc in `validate` because they are not implemented there.
- move the documentation to the `validates` method as it is referenced by individual validators
- as `allow_nil` and `allow_blank` are available for every subclasses `EachValidator`, added them as `:if`, `:on` etc. options
- move redundant docs for `allow_nil` and `allow_blank` except for special behaviors such as in `numericality.rb`