clarify array_contains function description

## What changes were proposed in this pull request?

The description in the comment for array_contains is vague/incomplete (i.e., doesn't mention that it returns `null` if the array is `null`); this PR fixes that.

## How was this patch tested?

No testing, since it merely changes a comment.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Will Manning <lwwmanning@gmail.com>

Closes #17380 from lwwmanning/patch-1.