docs: add SET BACKFILL_PARALLELISM clause to ALTER command documentation

[Copilot]

Documents the new SET BACKFILL_PARALLELISM clause added in risingwavelabs/risingwave#24323, which allows independent tuning of parallelism during the backfill phase of streaming jobs.

Description

Added SET BACKFILL_PARALLELISM sections to all relevant ALTER command pages, following the same pattern as existing SET BACKFILL_RATE_LIMIT and SET PARALLELISM sections:

• sql-alter-table.mdx — includes a note that Iceberg engine tables are not supported
• sql-alter-materialized-view.mdx
• sql-alter-index.mdx
• sql-alter-sink.mdx
• sql-alter-source.mdx

Each section covers syntax, parameter semantics (ADAPTIVE/0 or fixed integer), and SQL examples:

-- Fix backfill to 4 parallel workers
ALTER MATERIALIZED VIEW mv1 SET BACKFILL_PARALLELISM = 4;

-- Use all available units during backfill
ALTER TABLE t1 SET BACKFILL_PARALLELISM = ADAPTIVE;

Related code PR

risingwavelabs/risingwave#24323

Related doc issue

https://github.com/risingwavelabs/risingwave-docs/issues/

Checklist

• I have run the documentation build locally to verify the updates are applied correctly.
• For new pages, I have updated mint.json to include the page in the table of contents.
• All links and references have been checked and are not broken.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Document: feat: add support to alter backfill parallelism for streaming jobs</issue_title>
<issue_description>This issue tracks the documentation update needed for the merged PR risingwavelabs/risingwave-docs#24323.

Source PR URL: risingwavelabs/risingwave#24323
Source PR Merged At: 2026-02-24T08:37:08Z

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.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Document: feat: add support to alter backfill parallelism for streaming jobs #1021

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