Documentation/rv: Explain epoll and aborted sleeps

epoll_wait() is a valid sleeping reason for real-time tasks because it
uses PI-aware locking, but the rtapp sleep monitor documentation only
discusses clock_nanosleep() and futexes. Document it.

ABORT_SLEEP represents a task restoring TASK_RUNNING before entering
the scheduler. Since the task does not actually block, it becomes
runnable again without a wakeup sequence unsafe for real-time. Document
this behavior.

Signed-off-by: Chao Liu <chao.liu@processmission.com>
Reviewed-by: Gabriele Monaco <gmonaco@redhat.com>
Reviewed-by: Nam Cao <namcao@linutronix.de>
Link: https://lore.kernel.org/r/20260729081102.73138-1-chao.liu@processmission.com
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>