Fix inaccurate docs in active_model errors

The default value for the argument `message` in `ActiveModel::Errors#add` has a new behavior
since ca99ab2481d44d67bc392d0ec1125ff1439e9f94.

```
Before
  person.errors.add(:name, nil)
  # => ["is invalid"]
After
  person.errors.add(:name, nil)
  # => [nil]
```
I think RF is right, saying "no +message+ is supplied" should be enough to say that passing `nil` is still passing something. Therefore `:invalid` won't be used for the message when `nil` is given.