Fix doc: set_callback also accepts an array of if:

When Active Record calls `set_callback` inside `after_commit`,
[these lines of code](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/transactions.rb#L276)
pass an **array** of methods as the `:if` condition:

```ruby
options[:if] = Array(options[:if])
options[:if] << "transaction_include_any_action?(#{fire_on})"
```

That made me realize that anyone could pass an **array** of `:if` and `:unless`
conditions to `set_callback`, since Active Support transforms these conditions
into an array anyways in [these lines of code](https://github.com/rails/rails/blob/master/activesupport/lib/active_support/callbacks.rb#L365):

```ruby
@if      = Array(options[:if])
@unless  = Array(options[:unless])
```

Long story short, this commit updates the documentation of the `set_callback`
method to explain that arrays are also accepted.

It also replaces +false+ and +true+ with false and true, since any _falsey_ or
_truthy_ value will work.

[ci skip]

0 files changed, 0 insertions, 0 deletions