Improve doc for Window functions

[weiatwork]

Description

The doc should make it clear that the ORDER BY clause in an aggregate function is not supported, if the aggregate function is used as a window function.

For example, this works:

SELECT ARRAY_AGG(phone) OVER (PARTITION BY custkey) FROM customer

while this one fails:

SELECT ARRAY_AGG(phone ORDER BY phone) OVER (PARTITION BY custkey) FROM customer

line 1:59: PARTITION BY expression 'custkey' must be an aggregate expression or appear in GROUP BY clause

Additional context and related issues

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[mosabua]

In your example you have an ORDER BY as part of the parameter for the array_agg function. The docs for the array_agg function clearly indicate that you just pass in a parameter .. not a statement snippet.

The error message you see is probably misleading but this doc change is incorrect from all I can tell. Wdyt @kasiafi ?

[weiatwork]

Do you mean this by docs for array_agg? Yes, this doc may also need to be updated to describe the additional usage, as detailed here. It's not a statement snippet per se, but a special clause supported by array_agg.

[mosabua]

Looks good after a minor tweak. Thank you and sorry for the delay.