update TiDB 6MB limit (#6226)

[ti-srebot]

cherry-pick #6226 to release-5.0
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# In docs-cn repo:
git pr https://github.com/pingcap/docs-cn/pull/6239

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git push git@github.com:ti-srebot/docs-cn.git pr/6239:release-5.0-521f8612501c

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Signed-off-by: youjiali1995 zlwgx1023@gmail.com

What is changed, added or deleted? (Required)

TiDB introduced txn-entry-size-limit in v5.0 and added it to the doc #5158, but there are some outdated descriptions left.

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

• master (the latest development version)
• v5.0 (TiDB 5.0 versions)
• 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)?

• This PR is translated from:
• 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-srebot]

@youjiali1995 you're already a collaborator in bot's repo.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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