fix(aggregation): remove outdated nested aggregates limitation note

QuestDB now supports using aggregate functions as arguments to other
functions, so this warning is no longer accurate.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: bluestreak01

documentation/query/functions/aggregation.md

@@ -126,30 +126,6 @@ is documented in each function's description.
 
 :::
 
-:::note Nested aggregates not supported
-
-QuestDB does not support using aggregate functions as arguments to other
-functions. For example, this is not allowed:
-
-```questdb-sql
-SELECT datediff('d', min(timestamp), max(timestamp)) FROM trades;
-```
-
-Running it will result in the following error:
-
-`Aggregate function cannot be passed as an argument`
-
-You can work around this limitation by using CTEs or subqueries:
-
-```questdb-sql title="CTE workaround"
-WITH minmax AS (
-  SELECT min(timestamp) AS min_date, max(timestamp) AS max_date FROM trades
-)
-SELECT datediff('d', min_date, max_date) FROM minmax;
-```
-
-:::
-
 ---
 
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