Clarify Array#- and Array#difference documentation

Currently we are not explicit enough regarding the potentially confusing
behavior of `Array#-` and `Array#difference` when it comes to duplicate items
within receiver arrays.

Although the original documentation for these methods does use an array with
multiple instance of the same integers, the explanation for the behavior is
actually imprecise.

> removing any items that also appear in +other_ary+

Not only does `Array#-` remove any items that also appear in `other_ary` but
it also remove any instance of any item in `other_ary`.

One may expect `Array#-` to behave like mathematical subtraction or difference
when it doesn't. One could be forgiven to expect the following behavior:

```ruby
[1,1,2,2,3,3,4,4] - [1,2,3,4]
=> [1,2,3,4]
```

In reality this is the result:

```ruby
[1,1,2,2,3,3,4,4] - [1,2,3,4]
=> []
```

I hope that I've prevented this potential confusion with the clarifications
in this change. I can offer this as evidence of likeliness for confusion:
https://twitter.com/olivierlacan/status/1084930269533085696

I'll freely admit I was surprised by this behavior myself since I needed to
obtain an Array with only one instance of each item in the argument array
removed.

[Fix GH-2068] [ci skip]

From: Olivier Lacan <hi@olivierlacan.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66831 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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