docs: array_to_string missing 3rd 'null_string' argument in documentation

[Omega359]

Describe the bug

The array_to_string function has an optional third argument null_string that if provided will replace null. This is tested in the slt tests:

# array_to_string scalar function with nulls #2
query TTT
select array_to_string(make_array('h', NULL, NULL, NULL, 'o'), ',', '-'), array_to_string(make_array(NULL, 2, NULL, 4, 5), '-', 'nil'), array_to_string(make_array(1.0, NULL, 3.0), '|', '0');
----
h,-,-,-,o nil-2-nil-4-5 1|0|3

To Reproduce

No response

Expected behavior

Docs should reflect the actual functionality.

Additional context

No response