mysql-compatibility: update ZERO_DATE docs

[morgo]

First-time contributors' checklist

• I've signed Contributor License Agreement that's required for repo owners to accept my contribution.

What is changed, added or deleted? (Required)

The behavior of zero_date changed in master to be more mysql compatibility. This has not been backported.

There are some minor exceptions still, since DDL changes can still produce invalid dates (see: pingcap/tidb#21564 ). But I think these are minor, and it is worth updating the docs since the main workflow as described (INSERT/UPDATE) now matches MySQL.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

What is the related PR or file link(s)?

pingcap/tidb#20206

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[morgo]

PTAL @9547 @lzmhhh123

Thanks!

[9547]

/lgtm

[ti-srebot]

@9547, Thanks for your review. The bot only counts LGTMs from Reviewers and higher roles, but you're still welcome to leave your comments. See the corresponding SIG page for more information. Related SIG: docs(slack).

[TomShawn]

LGTM

[ti-srebot]

@9547, @TomShawn, @lzmhhh123, PTAL.

[morgo]

/merge

[ti-srebot]

@morgo Oops! This PR requires at least 2 LGTMs to merge. The current number of LGTM is 1

[Joyinqin]

/label translation/done

[Joyinqin]

/unlabel translation/doing