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]
```
cc @fxn @zzak @matthewd WDYT?