Fix inaccurate docs in active_model errors [ci skip]

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]
Merge pull request #18099 from robsonmarques/fix_active_model_errors_add_docs

Fix inaccurate docs in active_model errors [ci skip]