Add return type in LISTAGG documentation

[OneRaynyDay]

trino> SELECT TYPEOF(listagg(value, ',') WITHIN GROUP (ORDER BY value)) listagg_type
    -> FROM (VALUES 'a', 'c', 'b') t(value);
 listagg_type
--------------
 varchar
(1 row)

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[findinpath]

@OneRaynyDay thank you for the contribution.

Please do take the time to squash the commits into one and use a meaningful commit message:

Add return type in LISTAGG documentation

[OneRaynyDay]

Done, thanks!

[electrum]

Thanks!