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.

sql/core/src/main/scala/org/apache/spark/sql/functions.scala

@@ -2896,7 +2896,7 @@ object functions {
   //////////////////////////////////////////////////////////////////////////////////////////////
 
   /**
-   * Returns true if the array contains `value`
+   * Returns null if the array is null, true if the array contains `value`, and false otherwise.
    * @group collection_funcs
    * @since 1.5.0
    */