tiflash: Update tiflash note of importing data by Lightning (#4516)

[ti-srebot]

cherry-pick #4516 to release-4.0

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Signed-off-by: JaySon-Huang tshent@qq.com

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)

Update tiflash note about importing data by Lightning. Users can create TiFlash replica before importing data by using TiDB Lightning since v4.0.6

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

• master (the latest development version)
• 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): close Users could create tiflash replicas before importing data w/ lightning #4515, Lightning does not work with table with tiflash replica tidb-lightning#273, lightning fail to load data into a table with TiFlash replica tidb-lightning#391

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]

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