Add a known issue for TiFlash mpp.

[ran-huang]

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)

Add a known issue for TiFlash mpp.

Due to the file structure changes in v6.1, I'll cherry-pick to v5.3~v5.0 after I resolve conflicts in v5.4 cp PR.

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

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

• master (the latest development version)
• v7.2 (TiDB 7.2 versions)
• v7.1 (TiDB 7.1 versions)
• v7.0 (TiDB 7.0 versions)
• v6.6 (TiDB 6.6 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)
• v5.0 (TiDB 5.0 versions)

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

• This PR is translated from: Add a known issue for TiFlash mpp. docs-cn#14086
• Other reference link(s):

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

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has not been approved.

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

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[qiancai]

Rest
/lgtm

[ti-chi-bot]

@qiancai: Please do not delete or edit you lgtm type comment!

In response to this:

Rest
/lgtm

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[Oreoxmt]

/lgtm

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-06-06 10:56:21.078119435 +0000 UTC m=+2072.922754029: ☑️ agreed by qiancai.
• 2023-06-09 01:37:54.174865745 +0000 UTC m=+48688.422787568: ☑️ agreed by Oreoxmt.

[ran-huang]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ran-huang

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [ran-huang]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.1: #13815.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-5.4: #13816.