dm: add description about strict-optimistic-shard-mode

[GMHDBJD]

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[lance6716]

lgtm

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[LGTM Timeline notifier]

Timeline:

• 2023-06-20 02:58:19.681471938 +0000 UTC m=+71665.082722384: ☑️ agreed by hfxsd.

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[ran-huang]

/approve

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