Document: feat: allow user to force filter pushed into join in streaming query

[neverchanje]

This issue tracks the documentation update needed for the merged PR #21289.

Source PR URL: risingwavelabs/risingwave#21289
Source PR Merged At: 2025-04-08T16:51:14Z

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.