v8.5.5: br: add compatibility between log backup and PITR (#20002)

[ti-chi-bot]

This is an automated cherry-pick of #20002

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增加了日志备份和恢复的兼容性章节。

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[qiancai]

Suggested change

	从 v9.0.0 开始，当存在日志备份任务时，如果**同时满足**以下条件，则可以正常进行快照恢复 (`br restore [full|database|table]`)，并且恢复的数据可以被进行中的日志备份（下称“日志备份”）正常记录：
	从 v8.5.5 开始，当存在日志备份任务时，如果**同时满足**以下条件，则可以正常进行快照恢复 (`br restore [full|database|table]`)，并且恢复的数据可以被进行中的日志备份（下称“日志备份”）正常记录：

[qiancai]

Suggested change

	> 当恢复记录了快照（全量）恢复数据的日志备份时，需要使用 v9.0.0 及之后版本的 BR，否则可能导致记录下来的全量恢复数据无法被恢复。
	> 当恢复记录了快照（全量）恢复数据的日志备份时，需要使用 v8.5.5 及之后版本的 BR，否则可能导致记录下来的全量恢复数据无法被恢复。