[Flink 39717][docs] Document SET/RESET key handling for ALTER TABLE, CATALOG, MODEL and DATABASE

[raminqaf]

What is the purpose of the change

The reference docs for ALTER TABLE, ALTER CATALOG, ALTER MODEL, and ALTER DATABASE describe SET/RESET only as "override the old value with the new one". The actual converter behavior handles several edge cases (duplicate keys, unknown keys, empty lists, reserved keys) that are not documented and differ across the four DDLs. This PR documents that behavior so users can predict the outcome without reading the converter source.

Docs-only change. No code changes. Aligns with the per-DDL behavior already documented for ALTER MATERIALIZED TABLE in FLINK-39662 / FLINK-39663.

Brief change log

• docs/content/docs/sql/reference/ddl/alter.md: spell out behavior for ALTER TABLE, ALTER CATALOG, ALTER MODEL, ALTER DATABASE under SET and RESET.
• docs/content.zh/docs/sql/reference/ddl/alter.md: mirror in the Chinese docs.

Summary

DDL	SET empty	SET dupes	SET reserved	RESET empty	RESET unknown	RESET reserved
TABLE	no-op	last-wins+warn	—	reject	no-op	connector
CATALOG	no-op	last-wins+warn	—	reject	no-op	type
MODEL	reject	last-wins+warn	—	reject	no-op	—
DATABASE	no-op	last-wins+warn	—	n/a	n/a	n/a

Verifying this change

Docs-only update. No tests added. Documented behavior is covered by existing parser/converter tests for each DDL.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? docs (this PR is the documentation)

---

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Opus: 4.7

[flinkbot]

CI report:

• ee1a17f Azure: FAILURE

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build