doc: distinguish udaf input list from array(e.g. split output)

We said agg functions input is `list`, and split output is a list too. But they're not the same.

Look for documentation describing UDAF (User-Defined Aggregate Function) inputs and the split function output. The issue indicates a need to clarify that 'list' for aggregate functions is different from the list output by split. Search the codebase for existing documentation on these functions and their type signatures. The change is likely in markdown or comment files explaining SQL functions.

索引モデルが issue の本文から書いたものです。