Add union method to Array

I introduce a `union` method equivalent to the `|` operator, but which
accept more than array as argument. This improved readability, and it
is also coherent with the `+` operator, which has a similar `concat`
method. The method doesn't modify the original object and return a new
object instead. It is plan to introduce a `union!` method as well.

Tests and documentation are included.

It solves partially https://bugs.ruby-lang.org/issues/14097

[Fix GH-1747] [Feature #14097]

From: Ana María Martínez Gómez <ammartinez@suse.de>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64787 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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