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]
```
A couple of minor details:

`message` is not a noun. You can refer "the message" with an article if "message" goes in regular font, or to `message` without article. For example, a user may be logged in, but in a method RDoc you'd say "`user` is an instance of `User`" (not "the `user`").

Also, `nil` should go in fixed-width font.

So, it would be

```
# <tt>:invalid</tt> is assumed (but not if +message+ is +nil+).
```