Document: fix(agg): fix first_value and last_value to not ignore NULLs

[neverchanje]

This issue tracks the documentation update needed for the merged PR #19332.

Source PR URL: risingwavelabs/risingwave#19332
Source PR Merged At: 2024-11-13T04:38:51Z

If it is a major improvement that deserves a new page or a new section in the documentation, please check if we should label it as an experiment feature.

[stdrc]

Actually we need to document for each aggregate function whether it ignores NULLs or not, like in PG docs. I'll do this later.