shardddl: add unlock ddl lock doc for optimistic mode

[lichunzhu]

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What is changed, added or deleted? (Required)

DM optimistic mode has support shard-ddl-lock in v6.0. This PR will update the related documents.

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

• This PR is translated from:
• Other reference link(s): Optimistic: support unlock-ddl-lock for Optimistic Sharding DDL tiflow#4087

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[lichunzhu]

@GMHDBJD @TomShawn PTAL

[TomShawn]

@GMHDBJD @TomShawn PTAL

Is there an FD JIRA for this? I think this is expected to be tracked in JIRA and triaged by Cira and QA.

[GMHDBJD]

LGTM

[ran-huang]

这里建议用 <SimpleTab> 来分别显示悲观模式和乐观模式

[lichunzhu]

Is this fix correct?298fff8

[ran-huang]

改成了这样：3e398cf

官网会渲染为类似这样的效果：

[ran-huang]

/verify

[ran-huang]

@lichunzhu @GMHDBJD PTAL at this commit cc2a426. The rest of the PR LGTM.

[qiancai]

Removed the needs-cherry-pick-release-6.6 label because the v6.6 docs have been archived at https://docs-archive.pingcap.com/zh/tidb/v6.6 and will no longer receive new updates.

[qiancai]

Removed the needs-cherry-pick-release-7.0 label because the v7.0 docs have been archived at https://docs-archive.pingcap.com/zh/tidb/v7.0 and will no longer receive new updates.

[qiancai]

Removed the needs-cherry-pick-release-7.2 label because the v7.2 docs have been archived at https://docs-archive.pingcap.com/zh/tidb/v7.2 and will no longer receive new updates.

[qiancai]

Removed the needs-cherry-pick-release-6.1 label because the v6.1 docs have been archived at https://docs-archive.pingcap.com/zh/tidb/v6.1 and will no longer receive new updates.