Add Kernel#Array behavior with `nil` to the documentation

[YannIsCoding]

Kernel#Array returns an empty array when given nil. However, nil is retained when given an array containing nil. Unfortunately, this somehow surprising behavior is not documented.

Array(nil) # => []
Array([nil]) # => [nil]

Shall I open a PR to remedy this? :)

[gazeldx]

Hi @YannIsCoding ,
[nil].to_a # => [nil]

It is not surprising to me. The document is clear to me.