tiflash: Add note about mpp plan change

[qiancai]

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 default behavior of the generated plan to tiflash has been changed since v7.3.0, but we didn't mention it in the release note.

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.

For details, see tips for choosing the affected versions (in Chinese).

• master (the latest development version)
• v8.1 (TiDB 8.1 versions)
• v8.0 (TiDB 8.0 versions)
• v7.6 (TiDB 7.6 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 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)

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

• This PR is translated from: tiflash: Add note about mpp plan change docs-cn#17219
• Other reference link(s): planner: complete fall through into MPP task if up-layer can generate a cop tiflash  tidb#44324, https://asktug.com/t/topic/1020202

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]

[LGTM Timeline notifier]

Timeline:

• 2024-05-08 06:27:04.659066287 +0000 UTC m=+1029778.416201860: ☑️ agreed by hfxsd.

[qiancai]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiancai

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #17445.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.6: #17446.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.0: #17447.

[JaySon-Huang]

LGTM