sched/psi: Update poll => rtpoll in relevant comments

The PSI trigger code is now making a distinction between privileged and
unprivileged triggers, after the following commit:

 65457b74 ("sched/psi: Rename existing poll members in preparation")

But some comments have not been modified along with the code, so they
need to be updated.

This will help readers better understand the code.

Signed-off-by: Fan Yu <fan.yu9@zte.com.cn>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Peter Ziljstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/202310161920399921184@zte.com.cn