Add a description of Arr::add when the key exists and its value is null

[crhg]

Currently Arr::add is described as follows:

The Arr::add method adds a given key / value pair to an array if the given key does not already exist in the array:

However, the value is updated when the key exists and the value is null.

>>> Arr::add(['price' => null], 'price', 100)
=> [
     "price" => 100,
   ]

This PR adds a description of that case.

A PR has been submitted that corrects this behavior (laravel/framework #12960), and at that time it was judged as 'Since this is a behavior change will hold off on this.'.

It would be difficult to make change that breaks backwards compatibility, so I think it would be better to have a description of this behavior.

[taylorotwell]

Fixed.