Fix sometimes documentation

Unless I am missing something - it is not advisable to use `sometimes` and `required` validation rules together.

i.e. if the email is only validated `sometimes` if it exists in the `$data` - then `required` will never be fired if the value is not in `$data`. If the value *is* in `$data` - then you dont need the `required` field, because you know it is already there...?

Plus - if you actually do `require` something, then you should never really be using it with `sometimes` - its really a one or the other type of thing?