Fix inaccurate docs in active_model errors [ci skip]

[robsonmarques]

The default value for the argument message in ActiveModel::Errors#add has a new behavior
since ca99ab2.

Before
  person.errors.add(:name, nil)
  # => ["is invalid"]
After
  person.errors.add(:name, nil)
  # => [nil]

[rafaelfranca]

I think the current documentation already cover this.

If no +message+ is supplied, <tt>:invalid</tt> is assumed.

If you pass nil you are suppling a nil message.

[rafaelfranca]

cc @fxn @zzak @matthewd WDYT?

[zzak]

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.

[fxn]

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+).

[robsonmarques]

NIce. Thanks! 😄

[rafaelfranca]

Awesome! Thank you for working on this

[robsonmarques]

@rafaelfranca My pleasure!

[egilburg]

Speaking of returned value ["is invalid"] - is the exact return value significant? (as opposed of the side effect of adding the actual error to object)?

Because a structure like { name: ["is invalid"] }, or the errors object, or even the actual model instance, all seem like more sense to be returned here.

[zzak]

@egilburg I'm not sure what you mean..

@robsonmarques Thank you for the patch!

[robsonmarques]

@zzak You're welcome! 😄