array data type documentation enhancement

[xxchan]

https://risingwave-labs.slack.com/archives/C0881T5531D/p1750549763043189

From customer:

Documentation suggestion: I think there is an important missing part in the array data type documentation.
PostgreSQL offers an "array constructor" that takes a subquery. For example: select array(select 1);
BigQuery offers the same thing with the same syntax.
RisingWave does not document this in the data type page or on the array functions page. Yet it appears to work! I can run select array(select 1); and it seems to work ok...??