ADMIN CHECKSUM TABLE command undocumented

[morgo]

Change Request

This repository is ONLY used to solve issues related to DOCS.
For other issues (related to TiDB, PD, etc), please move to other repositories.

Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs.

The command added here is completely undocumented pingcap/tidb#6067

2. Describe your suggestion or addition.

I expect there would be some documentation here: https://pingcap.com/docs/sql/admin/#admin-statement

3. Provide some reference materials (documents, websites, etc) if you could.

As well as the commit referenced above, this command appears to be used by lightning.

[kennytm]

@morgo In #663 the admin show slow statement was documented in sql/slow-query.md instead of sql/admin.md. Do you think it is better to document admin checksum table in tools/lightning/*?

Additionally there are several undocumented ADMIN statements like admin check table, admin check index, admin cleanup index etc (not related to Lightning).

[morgo]

@kennytm I think they should all be documented :-) There is no harm in mentioning them in both places.

[morgo]

ADMIN is now documented here:
https://pingcap.com/docs/v3.0/reference/sql/statements/admin/

It makes sense to break this into sub-statement documentation though.

[ghost]

I will close this and handle in #2639