Feedback: Online Schema Changes - document specifically which changes cause rewrites/backfills

Page: https://cockroachlabs.com/docs/v25.2/online-schema-changes.html

## What is the reason for your feedback?

[X] Missing the information I need

[ ] Too complicated

[ ] Out of date

[ ] Something is broken

[ ] Other

## Additional details

Customers would like to be able to plan ahead, especially when doing schema changes on large tables, but they need more information than is currently provided in the docs.

This page mentions some operations that use a lot of disk space or are computationally expensive, but not in a very specific way. Please provide a complete list of schema changes that use a lot of disk space and/or are computationally expensive.

E.g. the page says

> **Estimate your storage capacity before performing online schema changes**
>
> Some schema change operations, like adding or dropping columns or altering primary keys, will temporarily increase a cluster's storage consumption. Specifically, these operations may temporarily require up to three times more storage space for the range size while the schema change is being applied, and this may cause the cluster to run out of storage space or fail to apply the schema change.

and

> **Run schema changes with large backfills during off-peak hours**
>
> Online schema changes that result in large backfill operations (for example, [ALTER TABLE ... ALTER COLUMN](https://www.cockroachlabs.com/docs/v25.2/alter-table#alter-column) statements) are computationally expensive, and can result in degraded performance.

Further discussion is in the #help-sql Slack channel starting 2025-May-22, including mention of an internal runbook that contains a list of schema changes that cause a backfill. This runbook page is at

https://cockroachlabs.atlassian.net/wiki/spaces/CKB/pages/3726966834/Playbook+Schema+Changes+With+Backfill

THANK YOU

Jira issue: DOC-13753