Add a known issue for TiFlash mpp. (#13734)

[ti-chi-bot]

This is an automated cherry-pick of #13734

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

[ran-huang]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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• OWNERS [ran-huang]

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[ran-huang]

/lgtm

[qiancai]

/lgtm

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-06-09 01:48:51.343233611 +0000 UTC m=+49345.591155437: ✖️🔁 reset by ran-huang.
• 2023-06-09 01:53:17.81745685 +0000 UTC m=+49612.065378673: ☑️ agreed by ran-huang.
• 2023-06-09 01:55:33.18727464 +0000 UTC m=+49747.435196454: ☑️ agreed by qiancai.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-5.3: #13822.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-5.0: #13823.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-5.1: #13824.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-5.2: #13825.