docs for N1QL operator ARRAY should include indexed example

The ARRAY operator for N1QL is documented here:

https://docs.couchbase.com/server/6.0/n1ql/n1ql-language-reference/collectionops.html#array

This operator has a two-parameter form, where the first index is not the array value, but rather its index. This usage should be demonstrated:

select array v for i:v in ["a", "b", "c", "d"] when i < 2 end

returns 

[

  {

    "$1": [

      "a",

      "b"

    ]

  }

]