Documentation: PM: Adjust freezing-of-tasks.rst to the freezer changes

The core freezer logic has been modified by commit f5d39b02


("freezer,sched: Rewrite core freezer logic"), so adjust the
documentation to reflect the new code. The main changes include:

 - Drop references to PF_FROZEN and PF_FREEZER_SKIP
 - Describe TASK_FROZEN, TASK_FREEZABLE and __TASK_FREEZABLE_UNSAFE
 - Replace system_freezing_cnt with freezer_active
 - Use a different example for the loop of a freezable kernel thread,
   since the old code is gone gone

Signed-off-by: Kevin Hao <haokexin@gmail.com>
[ rjw: Subject and changelog edits, doc text adjustments ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>