updated the `allow_nil` and `allow_blank` docs
- 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`

correct the font tag according to Rails contribution guides