Track v1.2 documentation follow-ups

## Scope

Track unfinished documentation work for user-facing changes included in `v1.2.0-beta.1`.

Requests whose upstream changes are not included in `v1.2.0-beta.1` are outside the scope of this issue.

## Documentation work

- [ ] #2025 — Document the SQL formatting HTTP API introduced by [GreptimeDB #6691](https://github.com/GreptimeTeam/greptimedb/pull/6691). Owner: @fengjiachun
- [x] #2717 — Fix sparse primary key encoding documentation for v1.2. Owner: @sunng87
- [ ] #2718 — Document v1.2 breaking changes in the upgrade guide. Owner: @fengjiachun
- [ ] #2719 — Document the streaming EXPLAIN ANALYZE HTTP API. Owner: @discord9
- [x] #2720 — Document Prometheus Remote Write v2 and native histograms. Owner: @shuiyisong
- [ ] #2721 — Document the MySQL object store backend. Owner: @fengys1996
- [ ] #2722 — Document the `max_row_group_row_count` table option. Owner: @evenyag
- [ ] #2723 — Add an Events query guide for `greptime_private.events`. Owner: @WenyXu
- [x] #2724 — Document the Kafka WAL `create_index` option and v1.2 default. Owner: @WenyXu

Each child issue contains its upstream source links, documentation contract, and acceptance criteria.

## Version and language scope

Unless a child issue explicitly narrows the scope, update:

- English Nightly
- Chinese Nightly
- English v1.2
- Chinese v1.2

## Completion criteria

- All tracked behavior is documented accurately for v1.2.
- Nightly and v1.2 documentation remain aligned where the behavior applies to both.
- English and Chinese content is aligned in meaning.
- Affected locale builds pass.
- All checklist issues above are closed.

Start with the linked child issues #2025 and #2718–#2723, each of which contains its source links, documentation contract, and acceptance criteria. Check the English and Chinese Nightly and v1.2 documentation, then verify affected locale builds and confirm every checklist item is closed.

Written by the indexing model from the issue text.