As pull request apache#17200 & apache#17368 introduced support for scheduling sleep, a documentation is needed for different sleep interfaces.
This patch adds the description for sleep interfaces currently provided in NuttX, including Scheduled sleep(nxsched_sleep()...), Signal-scheduled sleep(nxsig_sleep()...), and Busy sleep(up_udelay()).
Signed-off-by: Haokun Dong <donghaokun@lixiang.com>
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