Doc/PM: Sync with intel_powerclamp code behavior

Commit feb6cd6a ("thermal/intel_powerclamp: stop sched tick in forced
idle") changed how idle injection accouting, so we need to update
the documentation accordingly.

This patch also expands more details on the behavior of cur_state.

Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Reported-by: Wang, Xiaolong <xiaolong.wang@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>