Document: feat: let `ALTER SYSTEM set <session_parameter>` take effect in the current session

[neverchanje]

This issue tracks the documentation update needed for the merged PR #21294.

Source PR URL: risingwavelabs/risingwave#21294
Source PR Merged At: 2025-04-09T07:25:02Z

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.