update doc on wal-recovery-mode and use-direct-io-for-flush-and-compaction

[ekexium]

Signed-off-by: ekexium ekexium@gmail.com

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[TomShawn]

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[TomShawn]

/merge

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This pull request has been accepted and is ready to merge.

Commit hash: 8e28478

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #7025.